Size Structure of Exploited Holothurian Natural Stocks in the Hellenic Seas

Abstract

Size limitations are commonly applied as regulatory measures for the sustainable management of marine invertebrate fishery resources. However, the setting of biologically meaningful size limits in holothurians is puzzling, due to the limited knowledge of their biology and the great plasticity in size and weight, owing to the increased contractibility of their body, and the large quantity and variability of their coelomic fluid. To evaluate the efficiency of official size limits in Hellenic fishery regulation, the biometry of the exploited species, i.e., H. tubulosa, H. poli, H. mammata, and H. sanctori, was assessed in the main fishery grounds of the Hellenic Seas. Specimens of all four species were haphazardly collected and measured for total length and drained body weight at 46 sampling sites dispersed in the north Aegean, the Sporades, the Cyclades, the Dodecanese, and the Ionian fishery grounds. According to presented results, the official size limit of 180 g for drained weight appeared to be adequate for H. tubulosa and H. mammata. Adjustment of the relevant regulations for H. poli and H. sanctori are suggested by reduction to 140 g for the former and increment to 200 g for the latter species, to better fit their biological traits.

1. Introduction

Holothuroidea represents the most divergent class of echinoderms [1]. The various members of the class have lost the spines that nominate the phylum, and the calcareous endoskeleton has been limited to residual stereomatic ossicles embedded into the body wall. Instead of a robust test covered with spines, the somatic wall of holothurians is leathery and muscular. It consists of successive layers of different thickness: an external thin layer, the cuticle, followed by a thin epidermis, a thick dermis, which is the most developed layer, and a muscular layer of circularly arranged muscles interrupted by five longitudinal muscle bands that run the inner part of the body wall [2]. The dermis is mainly composed of connective tissue—constituting up to 60% of the body wall biomass—that contains a peculiar collagen type, the tonic collagen, which determines the texture of the body due to its mechanical mutability under nervous control [3]. Due to the structure of the body wall, the animal exhibits extremely high plasticity in form and stiffness, making holothurians’ biometry challenging.

Owing to the well-developed collagenous and protein-rich dermis, the body wall of holothurians has been traditionally explored for centuries in medicine and gastronomy, in Japan, China, and many other Southeast Asian countries, as “namako”, “hai shen”, “bêche de mer”, and “trepang” [4]. Today, the consumption and trade of holothurians’ body wall has expanded [5,6], whereas various products of sea cucumbers are included as high-priced nutraceuticals [7]. The market prices of sea cucumbers have sharply increased during the last years, providing significant income to local communities [4,8,9] and leading to a global expansion of their fisheries [10,11]. However, sea cucumber fisheries often follow a boom-bust-ban pattern [12,13] and the high demand for holothurians has caused the decline, or even the collapse of exploited populations worldwide [14], with several species included in IUCN red lists [15]. New species and grounds are persistently being sought [16] and the sustainable management of natural stocks to optimize economic yield is urgently needed [8,10,13,14,17,18].

Size limits are fundamental in fisheries management practices linking individual to ecosystem-based approaches [19]. However, the setting of optimum size limits in holothurians is puzzling, as the knowledge of basic biological parameters for most species is limited [14,18,20]. Additionally, holothurians exhibit high plasticity in size, owing to the increased contractibility of their body [21] derived by the peculiar structure of their body wall, as described above. Therefore, size limits based on length measurements are dubious for sea cucumber biometry-based management approaches [20]. Moreover, holothurians have a very developed somatic cavity fulfilled with coelomic fluid; seawater enters and expels periodically from the cloaca during respiration, or voluntarily by the animal, which dispels water under stress, for example when exposed to air [22]. Accordingly, the same individual exhibits highly different weights, mostly depending on the amount of water contained in the body cavity, causing difficulties in the estimations of biomass through weight measurements [20].

In the Hellenic Seas, the species Holothuria tubulosa Gmelin, 1793 and H. poli Delle Chiaje, 1824 constitute the main target of sea cucumber fisheries. Additional congeneric species are edible and locally explored in the Mediterranean, but not in Greece. However, H. mammata Grube, 1840 is sparsely collected, and the fishery of H. sanctori Delle Chiaje, 1823 may be profitable despite its handling difficulties due to Cuvier tubules [6]. Despite the limited data on above species biology [6], their fishery is regulated under the Presidential Degree 48/2018 that defines a daily quota and size restrictions on allowable catch; thus, each Holothuria spp. caught should be over 180 g in total wet weight. However, the precise estimation of weight is not always feasible, and this is a severe impediment in the applied management of their fisheries that creates conflicts and arguments between fishermen and authorities. The substitution of total weight with eviscerated weight will increase the precision of weight measurements [17,23,24], but the process demands the sacrifice of the animals which otherwise may have been returned into the sea with high survival potential.

Considering all of the above, the present study aims to assess the size structure of commercial Holothuria spp. in the main fishing grounds of the Hellenic Seas. On this basis, size–weight distributions are provided as essential information on the status of natural stocks. Different biometrical characters are used and through their relationships, the most reliable setting of size limits is proposed per target species for applied fisheries management.

2. Materials and Methods

The present study was carried out in the coastal waters of the Hellenic Seas, in the five main fishing grounds of Holothuria spp, i.e., north Aegean, Ionian, Sporades, Cyclades, and Dodecanese (Figure 1). Within each ground, a number of sampling sites were set as follows: 11 sampling sites in the north Aegean, 8 in the Sporades plateau—including the National Marine Park of Alonissos Northern Sporades (NMPANS), 10 in the Cyclades island complex, 10 in the Dodecanese island complex, and 7 in the Ionian Sea. Overall, 46 sites were sampled for commercial holothurians, located in 20 islands and 6 coastal bays (

Table 1. Location of sampling stations and sampling month, together with the number of collected (N) and eviscerated (eN) Holothuria spp. specimens to assess biometry. H.t. = Holothuria tubulosa, H.p. = Holothuria poli, H.s. = Holothuria sanctori, H.m. = Holothuria mammata.

Holothurians fisheries grounds in the Hellenic Seas		Location	Toponym	Station Longitude/Latitude	N/eN
					H.t.	H.p.	H.s.	H.m.
	North Aegean (April 2022)	Kavala Gulf	Agiasma	40°52.276′ N 24°36.347′ E	7	12/10
			Nea Peramos	40°50.268′ N 24°20.040′ E	10	15
		Thermaikos Gulf	NeaMoudania	40°13.770′ N 23°17.555′ E	221/20	49
			Potidea	40°11.572′ N 23°19.209′ E	41/20	90/20	2/2
		Toroneos Gulf	Nea Fokea	40°8.440′ N 23°23.238′ E	1		3/3
			Gerakini	40°15.962′ N 23°26.551′ E	29/20		1/1
		Limnos	Plaka	40°1.062′ N 25°26.820′ E	5	7	2/2
			Kotsina	39°56.999′ N 25°17.646′ E	3	5	1/1
			Moudros	39°52.476′ N 25°14.712′ E	6	12/10
		Lesvos	Kalloni	39°9.356′ N 26°12.159′ E	3	10
			Geras	39°2.524′ N 26°31.241′ E	2	5
	Sporades (May 2019)	Kyra Panagia *	Planitis	39°20.823′ N 24°04.495′ E	88/20	115/15	9/9
			Ag. Petros	39°18.929′ N 24°03.469′ E	11	37
		Alonissos **	Gerakas	39°16.441′ N 23°57.045′ E	10/10	46/5	2/2
			Peristera	39°11.825′ N 23°58.522′ E	3	5	1/1	8/8
		Skopelos	Skala	39°7.718′ N 23°44.369′ E	3/3	15/5	5/5	4/4
		Pagasitikos Gulf	Nies	39°6.949′ N 22°55.946′ E	15/10	24/10
			Traxili	39°9.504′ N 23°6.050′ E		10
			Akti Petras	39°9.199′ N23°11.633′ E	7/7	14/5
	Cyclades islands (June 2021)	Andros	Palaiopoli	37°48.675′ N 24°49.587′ E	3
		Paros	Naoussa	37°8.416′ N 25°13.688′ E	57/10	64/5	2
			Paroikia	37°5.549′ N 25°8.244′ E	47	60/5	4/4
			Alyki	36°58.508′ N 25°7.363′ E	67/10	96/15	2	3/3
			Piso Livadi	37°0.414′ N 25°15.185′ E	49		4/4
		Naxos	Plaka	37°2.357′ N 25°21.555′ E	21/20		2/2
			Kastraki	37°0.419′ N 25°21.529′ E		15	5/4
		Ios	Skala	36°43.241′ N 25°16.022′ E	22/20	67/20	1	1/1
		Milos	Arkoudes	36°46.235′ N 24°24.961′ E	19	27	4/4
		Serifos	Livadi	37°8.201′ N 24°31.910′ E	2	47/20	2/2
	Dodecanese (December 2019)	Agathonissi	Ag.Georgios	37°27.456′ N 26°59.548′ E	1/1	24/10
			Skala	37°27.356′ N 26°58.009′ E		1
		Arkoi	Marathi	37°21.989′ N 26°43.588′ E		14/10
		Patmos	Sapsila	37°18.797′ N 26°33.570′ E		25
			Groikos	37°18.069′ N 26°33.815′ E		85/20
		Leros	Xirokampos	37°6.335′ N 26°52.387′ E	1/1	2	3	9/9
		Kalymnos	Telendos	36°59.716′ N 26°55.408′ E			2
			Therma	36°56.313′ N 26°59.267′ E			12/10	3/3
		Pserimos	Vathi	36°56.153′ N 27°9.249′ E	1/1	2	19/10
		Plati		36°56.728′ N 27°5.573′ E			3
	Ionian (April 2022)	Amvrakikos Gulf	Aktio	38°56.134′ N 20°44.625′ E		8/5
			Ag. Nikolaos	38°52.472′ N 20°45.804′ E		15/15
		Messiniakos Gulf	Mantineia	36°58.916′ N 22°8.705′ E		7
		Sagiada Bay	Sagiada	39°36.900′ N 20°8.849′ E		18/10
		Kerkyra	Peleka	39°34.434′ N 19°48.840′ E		4	2/2
		Lefkada	Vassiliki	38°37.135′ N 20°35.819′ E		5	1/1
			Nydri	38°42.356′ N 20°42.979′ E		14/10

* NMPANS ZoneA, ** NMPANS ZoneB.

).

Samplings were made from May 2019 to July 2021—mostly during late spring–midsummer (see Table 1)—in depths down to 25 m by scientists and sea cucumber fishermen using the surface air supply diving method [25] and a licensed small-scale fishery boat. They included a random collection of sea cucumbers along a 10-min dive, applying standard commercial fishing practices [6]. Holothurian catches from each station were sorted according to species [26] and kept in seawater containers. Each specimen was taken out of the water, exposed to air for 5 min and measured for length (L) using an electronic calliper (0.01 precision). Then, it was 5-min drained on filter paper, and weighed for total weight (W) using an electronic scale (1 g precision). All measured specimens were immediately returned alive into the sea except for a randomly collected subsample of at least 20 specimens per sampling site and/or fishing ground, which was further processed, i.e., dissected, eviscerated, and weighed (0.1 g precision), to estimate eviscerated weight, eW [23,27,28].

Analysis of variance was applied to examine spatial differences (between fishing grounds and between MPAs and open to fisheries marine areas in the Sporades plateau ground) in biometric variables (L, W, eW) of holothurian species using the general linear model [29]. Prior to the analyses, data were tested for normality with the Anderson–Darling test, while the homogeneity of variances was tested with Cochran’s test. The Fisher LSD test was used for post hoc comparisons. ANOVAs were performed using the SPSS software package (IBM SPSS statistics v.25, IBM Corp, Armonk, NY, USA).

Size–frequencies distributions were constructed for each holothurian species per fishing ground. Length and weight distribution histograms created per 1 cm and per 10 g classes, respectively [17,23,28], were analyzed to assess the modal length and the modal weight using the SPSS software package (IBM SPSS statistics v.25, IBM Corp, Armonk, NY, USA).

L, W, and eW relationships were examined using linear regression analysis [23,28] for each Holothuria species by pooling data over the five fisheries grounds to enhance the robustness on analysis.

3. Results

3.1. Commercial Holothurian Species

Overall, 2304 specimens of edible and commercial holothurians were fished, belonging to four species, namely Holothuria mammata Grube, 1840, H. poli Delle Chiaje, 1824, H. sanctori Delle Chiaje, 1823, and H. tubulosa Gmelin, 1793. Holothuria poli (the black sea cucumber) constituted 59.68% of the total catch, followed by H. tubulosa (the brown sea cucumber) with 33.81%. The remaining two species, H. mammata and H. sanctori, were only occasionally fished constituting 2.4% and 4.08% of the total catch, respectively. The synthesis of total catch per fishing ground revealed the prevalence of H. tubulosa in the north Aegean, whereas H. poli dominates the other grounds (Figure 2).

3.2. Holothuria tubulosa

Overall, 779 individuals of H. tubulosa were collected from 36 sampling sites (see Table 1). Many specimens were fished from the north Aegean (326), the Cyclades (286), and the Sporades (136), in contrast with the Ionian and the Dodecanese grounds, where only 27 and 3 specimens, respectively, were caught.

The size spectra of the species ranged from 4.78 to 34.12 cm in length with a mean at 12.10 ± 3.91 cm, and from 13 to 429 g in weight, with a mean at 115 ± 73.3 g. Mean length showed significant variability between fishing grounds (ANOVA results, F = 22.34, p < 0.01); smaller specimens were caught from the north Aegean grounds and larger from the Dodecanese—however, as only 3 specimens were found, this result should be cautionary interpreted—and the Sporades (Figure 3). Mean weight also showed significant differences between fishing grounds (F = 35.25, p < 0.01) with heavier specimens in the Sporades and less heavy in the north Aegean and Cyclades (Figure 3). Excluding the very few specimens caught in the Dodecanese, H. tubulosa population had larger dimensions in the Sporades ground. This divergence partly emerged from the protected H. tubulosa population in the NMPANS (Kyra Panagia, zone A, and Alonissos, zone B sampling sites) which was constituted by similar-in-size but larger-in-biomass individuals (ANOVAs F = 0.13, p > 0.01, F = 9.84, p < 0.01 for L and W, respectively) compared with the sites (Skopelos island and Pagasitikos gulf) outside the marine protected area (Figure 4).

Length frequency and weight frequency distributions per fishing ground—with the exception of the Dodecanese, where frequency distributions were not constructed as only three specimens were caught—were unimodal (Figure 5). The bulk of the population constituted medium-sized individuals in the north Aegean, the Cyclades, and the Ionian grounds, where larger individuals were also present but in low frequencies. In the Sporades, the bulk of the studied population had a larger size and weight.

Overall, 193 individuals of H. tubulosa were eviscerated to assess their weight, eW (see Table 1). Eviscerated weight ranged from 32.69 to 188.85 with a mean at 89.81 ± 31.23 g. Mean eW showed significant differences between fishing grounds (F = 5.33, p < 0.01) with heavier specimens in the Sporades, Cyclades, and Dodecanese (Figure 6A)—but only three specimens were caught in the latter ground. By focusing on the Sporades fishing ground, significantly heavier holothurians were collected from the marine protected area, NMPANS stations, (F = 19.78, p < 0.01, Figure 6B), a pattern already observed considering the estimated total weight (W) of the species. In general, eW followed the same spatial pattern as W, with the divergence of the Cyclades, which showed increased eW compared with W. This result probably arises from between island differences, as the H. tubulosa population was smaller in size and weight at the Paros sampling stations as opposed to Naxos and Ios. The subsample of holothurians to estimate eW in the Cyclades derived from one out of the four stations of Paros (mean W = 84.3 ± 31.97) and from the stations of Naxos (192.4 ± 81.32) and Ios (319.7 ± 69.92), and thus, eW was overall increased over the Cyclades island complex.

The examined length/weight and weight/weight relationships followed negative allometry (

Table 2. Morphometric relationships of the 193 specimens of H. tubulosa subsample over the fishing grounds of the Hellenic Seas; a = intercept, b = slope, r = correlation coefficient, R² = percentage prediction.

Relation	Model	a	b	r	R2
eW/L	eW = aLb	10.30	0.79	0.70	49.83	−allometry (b < 3)
eW/W	eW = a + bW	33.11	0.33	0.94	90.23	−allometry (b < 1)

). A strong relationship was established between eW/W (correlation coefficient > 0.9) and a moderate one between eW/L. Accordingly, the drained weight of H. tubulosa appeared to be a good predictor of its eviscerated weight (i.e., the consumed part of the sea cucumber).

3.3. Holothuria poli

Overall, 1375 individuals of H. poli were collected from 38 sampling sites (see Table 1). A very large number of specimens were fished from the Cyclades (661), and a large number from the Sporades (266), the north Aegean (205), and the Dodecanese (172), in contrast with the Ionian grounds, where only 71 specimens were caught.

The size spectra of the species ranged from 4.99 to 27.32 cm in length with a mean at 10.54 ± 3.29 cm, and from 4 to 384 g in weight, with a mean at 105 ± 67.62 g. Mean length showed significant variability between fishing grounds (ANOVA results, F = 233.25, p < 0.01); smaller specimens were caught from the north Aegean grounds and larger from the Dodecanese and the Sporades (Figure 7A). Mean weight also showed significant differences between fishing grounds (F = 111.96, p < 0.01) following exactly the same pattern as length (Figure 7B). The H. poli population showed non-significant differences in body size between the protected (NMPANS) and the open to fisheries sites in Sporades ground (ANOVAs F = 1.21, p > 0.01, F = 0.09, p > 0.01 for L and W, respectively, Figure 8).

Length–frequency and weight–frequency distributions per fishing ground were unimodal (Figure 9). The bulk of the population constituted medium-sized individuals in the Sporades, the Cyclades, and the Ionian grounds; large-sized holothurians predominated in the Dodecanese in contrast with the north Aegean, where small-sized specimens prevailed.

Overall, 220 individuals of H. poli were eviscerated to assess their weight, eW, (see Table 1). Eviscerated weight ranged from 23.2 to 127.3 with a mean at 71.37 ± 21.19 g. Mean eW showed significant differences between fishing grounds (F = 26.48, p < 0.01) with heavier specimens in the Sporades, Cyclades, and Dodecanese (Figure 10A). Focusing on the Sporades fishing ground, no significant differences were observed in holothurian size between the marine protected area, NMPANS, and the rest of the stations, (F = 0.22, p > 0.01, Figure 10B). In general, eW followed the same spatial pattern as W, with the divergence of Cyclades, which showed increased eW compared with W, probably due to the same reasons—between-island differences—stated above for H. tubulosa. The subsample of H. poli to estimate eW in the Cyclades derived from the Paros stations, where the species weighed much less (mean W = 68.13 ± 28.70), and from Serifos (206.31 ± 50.61) and Ios (174.11 ± 57.62), where much heavier sea cucumbers occurred. Accordingly, eW was overall increased over the Cyclades island complex compared with W.

The examined length/weight and weight/weight relationships followed negative allometry (

Table 3. Morphometric relationships of the 220 specimens of H. poli subsample over the fishing grounds of the Hellenic Seas; a = intercept, b = slope, r = correlation coefficient, R² = percentage prediction.

Relation	Model	a	b	r	R2
eW/L	eW = aLb	8.76	0.82	0.73	54.54	−allometry (b < 3)
eW/W	eW = a + bW	35.09	0.24	0.89	80.61	−allometry (b < 1)

). A strong relationship was established between eW/W (correlation coefficient > 0.8) and a moderate one between eW/L. Accordingly, the drained weight of H. poli appeared to be a moderately good predictor of its eviscerated weight (i.e., the consumed part of the sea cucumber).

3.4. Holothuria sanctori

Overall, 94 individuals of H. sanctori were collected from 25 sampling sites (see Table 1). Most of the specimens were fished from the Sporades (17), the Cyclades (39), and the Dodecanese (24), in contrast with the north Aegean and the Ionian, where only 9 and 3 specimens, respectively, were caught.

The size spectra of the species ranged from 4.50 to 20.15 cm in length with a mean of 12.56 ± 2.27 cm, and from 60 to 237 g in weight, with a mean of 153 ± 50.19 g. Mean length showed non-significant variability between fishing grounds (ANOVA results, F = 1.96, p > 0.01, Figure 11A), in contrast with mean weight, which showed significant differences (F = 10.39, p < 0.01) with heavier specimens in the Sporades and the Ionian (Figure 11B). As very few specimens were caught from the Sporades fishing ground, and almost all of them from the NMPANS stations, it was not possible to look for differences in H. sanctori body size between the protected (NMPANS) and the open to fisheries sites.

Length–frequency and weight–frequency distributions per fishing ground were unimodal (Figure 12). The bulk of the population constituted medium-sized individuals in the Cyclades and the Dodecanese grounds, whereas larger sized holothurians prevailed in the Sporades. This pattern was much more pronounced in weight distributions (Figure 12B).

Overall, 70 individuals of H. sanctori were eviscerated to assess their weight, eW, (see Table 1). Eviscerated weight ranged from 51.6 to 106.7 with a mean at 82.21 ± 13.91 g. Mean eW showed significant differences between fishing grounds (F = 18.38, p < 0.01) with heavier specimens in the Sporades, Dodecanese, and Ionian (Figure 13). In general, eW followed the same spatial pattern as W.

The examined length/weight and weight/weight relationships followed negative allometry (

Table 4. Morphometric relationships of the 70 specimens of H. sanctori subsample over the fishing grounds of the Hellenic Seas; a = intercept, b = slope, r = correlation coefficient, R² = percentage prediction.

Relation	Model	a	b	r	R2
eW/L	eW = aLb	20.82	0.53	0.54	28.77	−allometry (b < 3)
eW/W	eW = a + bW	40.07	0.25	0.88	78.33	−allometry (b < 1)

). A rather strong relationship was established between eW/W (correlation coefficient > 0.7) and a weak one between eW/L. Accordingly, the drained weight of H. sanctori appeared to be a moderately good predictor of its eviscerated weight (i.e., the consumed part of the sea cucumber).

3.5. Holothuria mammata

Overall, 28 individuals of H. mammata were collected from 6 sampling sites (see Table 1). Most of the specimens were fished from the Sporades (12) and the Dodecanese (12), in contrast with the Cyclades grounds, where only 4 specimens were caught.

The size spectra of the species ranged from 9.63 to 25.06 cm in length with a mean of 16.41 ± 3.94 cm, and from 73 to 297 g in weight, with a mean of 172 ± 61.97 g. Mean length showed significant variability between fishing grounds (ANOVA results, F = 25.07, p < 0.01); smaller specimens were caught from the Cyclades grounds and larger from the Dodecanese—however, as only 4 specimens were found in the Cyclades, this result should be cautiously interpreted—(Figure 14A). Mean weight also showed significant differences between fishing grounds (F = 6.53, p < 0.01) with heavier specimens in the Sporades and less heavy in the Cyclades and Dodecanese (Figure 14B).

As only 12 specimens were caught from the Sporades fishing ground, and most of them (8 specimens) from the NMPANS stations, it was not possible to look for differences in H. mammata body size between the protected (NMPANS) and the open to fisheries sites.

Length–frequency and weight–frequency distributions per fishing ground were unimodal (Figure 15). The bulk of the population constituted medium-sized individuals in length in the Sporades and of large-sized in the Dodecanese grounds (Figure 15A). Considering weight distributions, larger holothurians predominated in both above grounds (Figure 15B).

Overall, all 28 individuals of H. mammata were eviscerated to assess their weight, eW, (see Table 1). Eviscerated weight ranged from 65.1 to 131.4 with a mean of 92.42 ± 19.14 g. Mean eW showed significant differences between fishing grounds at 95% confidence level (F = 3.66, p = 0.04) with heavier specimens in the Sporades and Dodecanese (Figure 16), following the same general pattern as W.

The examined length/weight and weight/weight relationships followed negative allometry (

Table 5. Morphometric relationships of the 70 specimens of H. mammata subsample over the fishing grounds of the Hellenic Seas; a = intercept, b = slope, r = correlation coefficient, R² = percentage prediction.

Relation	Model	a	b	r	R2
eW/L	eW = aLb	20.17	0.54	0.67	43.83	−allometry (b < 3)
eW/W	eW = a + bW	48.33	0.26	0.83	68.07	−allometry (b < 1)

). A rather strong relationship was established between eW/W (correlation coefficient > 0.8) and a weak one between eW/L. Accordingly, the drained weight of H. sanctori appeared to be a moderately good predictor of its eviscerated weight (i.e., the consumed part of the sea cucumber).

4. Discussion

Marine invertebrates used to represent a very small proportion of worldwide fisheries production, made up mostly of cephalopods, shrimps, lobsters, and bivalves [12]. In recent decades, however, under the shrinking of finfish landings, invertebrate catches have been expanding, surpassing 10 million metric tons annually, and adding new target species [30]. Accordingly, their contribution to global fisheries production is increasing. As marine invertebrates have different biological traits that generate maximum sustainable yield at lower levels of harvest than finfish, and play a key functional role in marine ecosystems, the sustainable management of their fisheries and the safeguarding of ecosystem functioning is highly challenging [30].

Among newly explored invertebrates, holothurians denote a peculiar case, as many species have been traditionally explored by artisanal fisheries and at local level in Asian countries for over centuries [12]. In recent years, however, holothurian fishery has amplified and intensified, due to a strong demand for the final product [4,10,16]. They still represent, however, a small portion of global invertebrate catches [30]. Today, some of the explored grounds have either collapsed or been fully exploited, while some others are expanding [5,12], and up and down trends for specific species have been widely reported [12,14]. In the Mediterranean Sea, holothurians constitute a new target fishery resource that has been explored during at least the last 20 years [6,16]. Despite the augmented scientific interest in sea cucumber fisheries, their biology remains largely understudied and daily quotas unregistered.

In various parts of the world, including the Mediterranean Sea, the regulation of holothurian fisheries has been attempted with varying degree of success, but most often ineffectively [14]. This failure is generally attributed to the lack of biological data on fundamental life-history parameters such as reproduction, recruitment, and growth rate, together with their “accessibility easiness” due to their predominance on intertidal and shallow subtidal habitats, and their longevity that makes sea cucumbers especially susceptible to overfishing. Among regulations, limits on licenses and permitted fishing grounds predominate, whereas size limits are of particular importance as, combined with daily quotas (and so, population density), they may safeguard reproductive capacity and stock viability [14]. However, the setting of biologically meaningful size limits in the soft bodies and highly stiffing holothurians is puzzling [24].

By focusing on the Hellenic fisheries, the existing regulations recommend a size limit of 180 g for total wet weight for all exploited holothurian species (Holothuria spp). This setting was based on the limited available biometrical data of the species H. tubulosa [23,28] derived from the Sporades and the Dodecanese ground, as there is no information at all for the other edible species of the genus. In the present study, most specimens were much smaller (overall mean 115 ± 73.3 g) than the official size limit and exhibited strong variability between fishing grounds. More specifically, the mean size of H. tubulosa population was at the size limit in the Sporades ground, mostly due to the heavy—large in biomass—specimens caught in the NMPANS, and in the Dodecanese; however, only three specimens of the species were found in the latter area as opposed to previous samplings (2006–2008 period) [28]. The above comparisons present strong evidence of overfishing, as the mean weight of H. tubulosa overall decreased in the Hellenic fishing grounds and the weight spectra shifted towards smaller sea cucumbers (left-skewed distribution), except for the NMPANS marine protected area. Moreover, the population collapsed in the north Dodecanese ground, an area long ago explored for holothurians as fishing baits and presently for human consumption, where only three, albeit large-sized, individuals were found.

In the previous study of H. tubulosa in the Dodecanese [28], mean weight (104.6 ± 44.7 g) was also far lower than the official size limit, in contrast with the Pagasitikos Gulf in the Sporades ground [23] where it was much higher (218.33 ± 84.79). Considering the practical difficulties involved in the estimation of sea cucumber biomass, mostly due to water contained in the body cavity [20], most researchers suggest the application of eviscerated weight instead of drained weight [17]. The mean eviscerated weight of H. tubulosa (89.8 ± 31.2 g) over the Hellenic fishing grounds was higher than the relevant one previously reported for the Dodecanese (61.9 ± 23.2 g) [28], but smaller than the one formerly reported for the Sporades (108.5 ± 35.1 g) [23]. Eviscerated weight of H. tubulosa exhibited a strong relationship with drained weight, and accordingly, drained weight may be used as a proxy to estimate the biomass of the species. The relevant relationship with length was moderate; thus, size restrictions seem to be more reliable and accurate when based on weight [17,23] than on length of sea cucumbers, due to their high body plasticity [20,21,22]. However, both the size and weight structure of H. tubulosa are highly variable between different biogeographic areas of the Mediterranean Sea, as showed by González-Wangüemert [17], with a decreasing trend from the western towards the eastern basin. These authors used eviscerated weight as a proxy of H. tubulosa biomass; thus, comparing their results with the relevant from the present study, the species population in the Hellenic Seas (89.81 ± 31.23 g) seems to have similar biometrical features to the population from Ischia (91.45 ± 26.66 g). The biomass of H. tubulosa was much lower in the overexploited Turkish part of the Aegean Sea (Kusadasi, 58.69 ± 19.52 g), whereas the lowest values (28.64 ± 6.48 g) were reported from the oligotrophic Libyan Sea, in the south Crete [17]. Accordingly, the studied population seems to be under moderate fishing pressure, based on its size structure and environmental conditions—oligotrophic to mesotrophic water bodies of the Aegean Sea [31]. Considering differences in eviscerated weight between the fishing grounds of the Hellenic Seas, H. tubulosa exhibit decreased weight in the Ionian (71.94 ± 17.09 g) and the north Aegean Sea (80.6 ± 27.64 g). Mean eviscerated weight had similar values in the Sporades (98.54 ± 27.68 g) and the Cyclades (96.91 ± 36.64 g) grounds and increased values in the Dodecanese (105.67 ± 18.52 g). The shallow depth of sampling stations may explain the decreased weight of holothurians in the mesotrophic waters of the Ionian and the north Aegean grounds. In these grounds, fishermen collect holothurians from less than 5 m in depth, and a depth segregation pattern of H. tubulosa with smaller individuals in shallower and larger and heavier in deeper coastal waters has been documented [23,27]. Finally, significantly heavier holothurians were caught from the NMPANS marine protected area, highlighting once again the beneficial conservation status ensuing H. tubulosa sustainability [6].

Considering the species H. poli, H. sanctori, and H. mammata, there are no relevant data from the Hellenic Seas. The species were not formerly exploited as fishing bait, and their biology has never been studied before. Therefore, the presented results may only be compared with relevant data from other Mediterranean regions [17,32,33,34].

The studied H. poli population was composed of relatively large-sized individuals, which were, however, much smaller (overall mean 105 ± 67.62 g) than the minimum allowable catch size. Holothuria poli is smaller than other commercial Mediterranean species of the genus Holothuria and accordingly, a different, species-specific size limit should be set. Mean eviscerated weight of H. poli has been studied in four Mediterranean areas [17], revealing a decreasing trend in weight towards the eastern Mediterranean basin, as also reported for its congeneric H. tubulosa (see above). This decreasing trend has been attributed to the lower nutrient concentrations of the warm and saline eastern Mediterranean waters and to overfishing [16,17]. In the present study, the mean eviscerated weight of H. poli (71.37 ± 21.19 g) was similar to the population from Girona (71.39 ± 26.42), and much higher than the populations from Ischia (41.06 ± 11.41), Kusadasi (42.40 ± 11.84), and Crete (33.42 ± 9.75). The reduced size in Kusadasi may have resulted from overfishing, whereas oligotrophy may explain the decreased size in Crete, as was the case for H. tubulosa, as well. These results suggest that fishing pressure is still low and has not overall affected H. poli in the Hellenic grounds. Smaller specimens were caught from the north Aegean grounds, probably for the same reasons as for H. tubulosa, i.e., depth-segregation pattern, and somewhat smaller from Paros island, possibly due to the intense fishery prior to the COVID-19 epidemic [6], whereas the species population had similar biometrical traits between the NMPANS marine protected area and the open to fishery grounds. Considering all the above, the minimum allowable size should be reduced to fit the species biology.

Both the studied H. sanctori and H. mammata populations are occasionally fished in the Hellenic seas [6]. Their populations were constituted of moderate- to large-sized individuals, and the mean weight was a little smaller that the official size limit of 180 g for both species. Mean eviscerated weight was 82.21 ± 13.91 g and 92.42 ± 19.14 g, respectively. These values are close to the populations from Girona (96.1 ± 30.91) and Kusadasi (85.97 ± 21.41) for H. mammata [17] and were smaller than the relevant reported from Canaria (100.06 ± 23.61)—there are no data from the Mediterranean—for H. sanctori [35]. Therefore, fishing pressure does not seem to have affected the species populations so far in the Hellenic grounds.

Weight–frequency distributions were unimodal for all four species, as previously reported from the Sporades and Dodecanese grounds [23,28] and from other Mediterranean areas [17] and attributed to the reproductive traits and the absence of juveniles due to sampling limitations [35]. All examined morphometric relationships, i.e., eW/L and eW/W, followed negative allometry, and, thus, the relative growth of the body wall is lower than the relevant in length or drained weight, in accordance with similar studies [17,23,35].

The setting of biologically meaningful size limits is essential for sea cucumber regulative management [24]. Nevertheless, such attempts are tricky due to the plasticity of holothurian body dimensions and the limited biometrical data available for each commercial species [17]. As a stepping stone, minimum size should be at least larger than the size at maturity, as emphasized by Bruckner [36], to ensure the reproductive success of the exploited population and the maintenance of genetic diversity. Unfortunately, very little information exists for Mediterranean exploited Holothuria spp. size at maturity. According to Kazanidis et al. [23], 50% of the H. tubulosa population in the Aegean (Pagasitikos Gulf, Sporades ground) was mature at approximately 220 g drained weight, whereas according to Pasquini et al. [35], the relevant size was 14 and 15 cm in length (extended, relaxed length) for male and females, respectively. These values correspond to an eviscerated weight of about 60 g. Therefore, the existing size limits are close to the size at maturity of H. tubulosa, though the gathering of additional data from other Mediterranean locations is more than necessary. Considering H. poli, the relevant size at maturity is at 13.7 cm for relaxed length and 18 g for eviscerated weight [33]. Therefore, the species seems to be able to reproduce at relatively smaller sizes than H. tubulosa [35] and H. sanctori [34]. The latter species seems to reproduce at much larger body size, estimated at 20–21 cm and 100–110 g of eviscerated weight [34]. Lastly, H. mammata can reproduce at 14 to 16 cm [33], which corresponds to about 90 g for eviscerated weight. It should be noted, however, that all of the above estimations derive from very limited data, and so, specific studies focusing on the reproductive biology of exploited Holothuria spp. are urgently needed for the sustainable management of their fisheries. According to the above data, the official size limit seems to be adequate for the species H. tubulosa and H. mammata. The former species seems to be under moderate fishing pressure in the Hellenic grounds, as it has been collected as fishing bait for many years and intensively for human consumption during the last ten years, whereas the latter has just started to be explored. The relevant regulations for H. poli and H. sanctori should probably be adjusted according to their biological traits. More specifically, the size limit of H. poli is suggested to be reduced to about 140 g for drained weight, whereas the size limit of H. sanctori may be increased to 200 g.

5. Conclusions

In conclusion, this study reports the first comprehensive results on the biometry of edible and exploited Holothuria spp. over the Hellenic Seas. The presented results are useful to establish appropriate species-specific size limits and to efficiently manage natural stocks exploitation. They also underline the importance of the NMPANS marine protected area as a natural reservoir ensuring the viability of holothurian populations. The proposed size limits are based on the precautionary approach, which has been proven to be more effective in conserving sea cucumber natural stocks through a substantial price premium for larger holothurians. The analyzed Holothuria spp. biometry can also serve as a reference baseline for future comparisons to evaluate fishery effects on natural stocks, adjusting management measures to ensure sustainability.