Reasons for the Decline in Catches as Revealed by Long-Term Data from the Actual Operation of the Sea Urchin Fishery Hemicentrotus pulcherrimus in Mikuni, Fukui Prefecture, Japan

Abstract

Dive fisheries are widely practiced in coastal areas worldwide owing to their minimal equipment requirements and extensive historical background. Female divers in Korea and Japan have extensive knowledge of the local coastal environment and engage in sustainable fishing practices. However, the number of divers and their catches has been declining. In this study, long-term catch and effort data were collected to investigate the reasons for the decline in the catch of the sea urchin, Hemicentrotus pulcherrimus, in Mikuni, Sakai City, Fukui Prefecture, Japan. A significant correlation was observed between catch and effort, and the main reason for the decline in catch was a reduction in fishing effort. Fishermen have voluntarily limited their fishing efforts to prevent the depletion of natural stocks, and a decrease in the number of fishermen has contributed to this reduction. In addition, regulations on catchable size and fishing season were implemented. However, natural stocks appear to have declined. Although sea urchin fisheries have been sustainably practiced for a long time, current fishery management and regulations appear inadequate. Therefore, sustainable fishing requires identifying the factors that affect stock fluctuations and updating management practices.

1. Introduction

The 2030 Agenda for Sustainable Development [1], adopted by all United Nations member states in 2015, serves as a comprehensive blueprint aimed at promoting peace and prosperity for people and the planet, both in the present and future [2]. “Conserve and sustainably use oceans, seas, and marine resources for sustainable development” is aimed at sustainable development goal (SDG) 14, and “By 2020, effectively regulate harvesting and end overfishing, illegal, unreported and unregulated fishing and destructive fishing practices and implement science-based management plans to restore fish stocks in the shortest time feasible, at least to levels that can produce maximum sustainable yield as determined by their biological characteristics” is aimed at SDG 14.4 [3]. Cooperation with many stakeholders, including fishermen and researchers, is required to achieve these goals.

Dive fishing in shallow water has been practiced since ancient times because it does not require special mechanical equipment, and fishing grounds are easily accessible from land. For example, female divers in Jeju in the Republic of Korea and Japan practice dive fisheries. They are called “haenyeo” in Korean and “ama” in Japanese. They possess comprehensive knowledge of coastal resources and the environment and continue to fish sustainably [4,5]. Dive fisheries in shallow waters use local natural resources, which are strongly influenced by the abundance of stocks and the environmental characteristics of the local area. Traditionally, the natural environment and climate have influenced local people and their culture. Dive fisheries are community-based, resulting in regional diversity in fishing practices. Therefore, effective strategies for achieving sustainable fisheries must be tailored to account for their unique local environmental conditions and cultures.

Nakano et al. (2024) [6] reported that female divers in Mikuni, Sakai City, Fukui Prefecture, Japan, who fished for the sea urchin Hemicentrotus pulcherrimums between 2020 and 2022, assessed the local stock abundance through their fishing activities and voluntarily stopped fishing after capturing a specific quantity of sea urchins relative to their perceived abundance. This indicates that a mechanism exists in this region to prevent the depletion of local sea urchin stocks due to overfishing. This voluntary fishery management was controlled by a branch of the local fisheries cooperative, which has exclusive rights to fish for sea urchins in the area. This system is an example of how fishermen are already implementing the SDGs of “sustainably use the marine resources” and “regulate harvesting”.

The sea urchin H. puclcherrimus has been harvested in the coastal areas of Fukui Prefecture since at least 1601 [7]. The target size of the sea urchin exceeds 2 cm in test diameter, and the fishing season is from 21 July to 20 August, according to Fukui Prefecture Fisheries Adjustment Regulations [8]. The test diameter of the sea urchin reaches 24–25 mm at the age of 2 years [9], and the spawning season is from December to April [10]. Brown and red algae have been identified in the gut contents of sea urchins, and macroalgae are their primary food source [11]. However, the catch of sea urchins has been declining. Additionally, the number of female divers (“ama”) engaged in sea urchin fishing has declined, but the relationship between both declines has not yet been verified.

In Japan, the number of fishermen continues to decline, decreasing from 238,000 in 2003 to 123,000 by 2022. Furthermore, individuals aged >65 years accounted for 37.7% of the total in 2022 [12]. In Jeju Island, South Korea, more than 85% of female divers (“haeneyo”) are estimated to be >50 years old, and the number of divers has decreased from 23,000 in 1965 to ˂4000 in 2017 [4]. Dive fishing is practiced without any mechanical equipment; therefore, a decrease in the number of divers should affect the amount of catch and the exploitation of natural stocks. However, no information is available on the actual fishery situation.

“Echizen-uni” has long been considered a local specialty made from the gonads of the sea urchin H. pulcherrimus [13] and is of considerable interest to the fishing industry. Consequently, long-term data on the operation of the fishery have been preserved over the years. This study aimed to collect data on catch, the number of fishing operators in the sea urchin fishery, the number of fishing days, the total number of fishing operators, and catch per unit of effort (CPUE), as well as to clarify the relationship between catch and fishing operations in a sea urchin fishery in Mikuni, Sakai City, Fukui Prefecture, Japan. Furthermore, the results were used to examine the state of the voluntary management of the sea urchin fishery and assess the impact of the declining number of fishermen on the fishery. The challenges in achieving sustainable sea urchin fisheries are also discussed.

2. Materials and Methods

2.1. Long-Term Trend in Catch (Raw Gonad Weight of the Sea Urchin H. pulcherrimus)

The Oshima Fisheries Cooperative is located in Mikuni, Sakai City, Fukui Prefecture, Japan, and the sea urchin H. pulcherrimus fishery operates in the following four branches of the cooperative: Komegawaki, Anto, Saki, and Kaji (Figure 1). I investigated the catch data of sea urchins and used the records reported by Kawana (1938) [10], Minamisawa and Ogawa (1959) [9], and Maekawa (1971) [14] from 1925 to 1937, 1948 to 1952, and 1953 to 1970, respectively. The weights of the catches were recorded in the traditional Japanese unit of weight, “kan”, as reported by Minamisawa and Ogawa (1959) [9]. For consistency, these records were converted to the “kg” unit according to the following relationship: 1 kan = 3.75 kg. The salted gonad weights of the catches were converted into raw gonad weights using the following formula: (raw gonad weight) = (salted gonad weight)/0.855 [15]. All catch data in this study are represented as the raw gonad weight of sea urchins. As described below, the fisheries cooperative monitors the amount of catch by purchasing the harvested sea urchins. All the catch data used in this study were originally derived from the fisheries cooperative.

2.2. Catch, Number of Fishing Operators, Fishing Days, Total Number of Fishing Operators, and CPUE

Saki and Anto, branches of the Oshima Fisheries Cooperative, purchased all raw gonads of sea urchins caught in the fishing grounds managed by each branch. The preserved sea urchin purchase records from various branches were used to compile the catch (raw gonad weight), number of fishing operators, fishing days, and total number of fishing operators yearly. Some of the same data from Kaji and Komegawaki were preserved at the Oshima Fisheries Cooperative Office and tabulated each year. Additionally, from 2020 to 2022, logbooks were requested from all fishermen who fished for sea urchins at the study sites and were collected and tabulated similarly. The logbooks used in this study were similar to those used by Nakano et al. (2024) [6]. The annual CPUE was calculated using the following formula: CPUE [g/day/fishing operator] = (catch)/(total number of fishing operators).

2.3. Data Analyses

All analyses were conducted using R (version 4.4.2) [16] and RStudio (version 2024.12.0+467) [17]. Data formatting, plots, statistical analyses, and map making were conducted using the tidyverse R (version 2.0.0) [18], lubridate R (version 1.9.3) [19], sf R (version 1.0.16) [20], ggspatial R (version 1.1.9) [21], and ggpmisc R packages (version 0.5.6) [22]. Map data (shape files) were obtained from the websites of the Ministry of Land, Infrastructure, Transport, and Tourism of Japan [23] and Natural Earth [24].

To determine the factors affecting catch, the correlation between fishing effort (total number of fishing operators) and CPUE was investigated. In the analysis, catch efficiency was assumed to remain stable among fishermen. Since fishing was conducted without a mechanical apparatus, catch efficiency remained constant over time. The catch efficiency of dive fisheries generally varies depending on factors such as the amount of time fishermen can hold their breath. However, because the fishing grounds are shallow (depths ranging from 0.5 to 3 m), breath-holding time does not significantly impact efficiency [6]. Therefore, the analysis was conducted under the assumption that catch efficiency does not differ among fishermen. Additionally, since all fishermen operated for the same duration, their catch efficiency was assumed to be equal.

In this study, a statistically significant relationship was considered to exist when the p-value was less than 0.05.

3. Results

3.1. Catch Trend of the Sea Urchin H. pulcherrimus in Mikuni

Catch data were obtained fragmentally from 1925 to 1970 and annually after 1979 (Figure 2). The catch fluctuated and declined over time, with considerable fluctuations in catch in Saki between 1925 and 1937, Anto between 1960 and 1970, and Anto between 1979 and 1995.

The catch after 2010 is shown in Figure 3. The catch also fluctuated annually after 2010.

3.2. Trends of Catch, Number of Fishing Operators, Fishing Days, Total Number of Fishing Operators, and CPUE

The annual catch, number of fishing operators, fishing days, total number of fishing operators, and CPUE are shown in Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8. Empty years indicate when no data were available. Some catch data from 1980 to 1990 differ between Figure 2 and Figure 4. Although all catch data are derived from the fisheries cooperative, the reason for the differences is unclear.

The catch fluctuated and declined (Figure 2 and Figure 4). The number of fishing operators continued to decline; however, the decline in Saki and Kaji slowed in 2010 (Figure 5).

The number of fishing days decreased, with a large interannual variation (Figure 6). In years with a low number of fishing days (Figure 6), annual operations at all branches were completed within four days.

The total number of fishing operators declined with annual fluctuations. Only fragmentary data were available for Kaji and Komegawaki (Figure 7).

The CPUE at Anto was approximately 500 g/day/fishing operator from 1979 to 1992. Thereafter, the CPUE decreased over many years, with large annual variations. The CPUE in Saki varied yearly but increased in some years (Figure 8).

3.3. Relationship Between Catch and Total Number of Fishing Operators

The relationship between catch and the total number of fishing operators is shown in Figure 9. The linear regression formula, coefficient of determination, correlation coefficient, and p-values calculated from these relationships are listed in

Table 1. Regression table showing relationship between catch and total number of fishing operators.

Branch	Linear Regression Formula	Coefficient of Determination	Correlation Coefficient	p-Value
Anto	y = 0.544x + −30.192	0.962	0.889	0.000
Saki	y = 0.44x + 4.114	0.930	0.847	0.000
Kaji	y = 1.222x + −86.263	0.948	0.200	0.917
Komegawaki	y = 0.354x + −4.749	0.999	0.800	0.333

. The correlation coefficients were 0.889 for Anto and 0.847 for Saki, indicating a significant correlation (p < 0.05, Spearman’s rank correlation test). The correlation coefficients were 0.200 for Kaji and 0.800 for Komegawaki, indicating no significant correlation (p > 0.05, Spearman’s rank correlation test).

3.4. Relationship Between CPUE and Total Number of Fishing Operators

The relationship between the CPUE and the total number of fishing operators is shown in Figure 10. The linear regression formula, coefficient of determination, correlation coefficient, and p-values calculated from these relationships are listed in

Table 2. Regression table showing relationship between CPUE and total number of fishing operators.

Branch	Linear Regression Formula	Coefficient of Determination	Correlation Coefficient	p-Value
Anto	y = 0.363x + 255.538	0.311	0.617	0.005
Saki	y = −0.004x + 473.773	0.000	0.284	0.190
Kaji	y = 3.741x + −17.163	0.707	0.200	0.917
Komegawaki	y = 0.554x + 145.257	0.899	0.400	0.750

. The correlation coefficient was 0.617 for Anto, indicating a significant correlation (p < 0.05, Spearman’s rank correlation test). The correlation coefficients were 0.284 for Saki, 0.200 for Kaji, and 0.400 for Komegawaki, indicating no significant correlation (p > 0.05, Spearman’s rank correlation test).

4. Discussion

In this study, the long-term catch trend of the sea urchin H. pulcherrimus in Mikuni, Sakai City, Fukui Prefecture, Japan, after 1925 was examined. Additionally, data on the changes in catch, number of fishing operators, number of fishing days, total number of fishing operators, and CPUE from 1979 to 2022 were comprehensively examined. The catch of sea urchins has declined over the decades (Figure 2), and the catch, number of fishing operators, and total number of fishing operators have decreased since 1979 (Figure 4, Figure 5 and Figure 7). The number of fishing days decreased, exhibiting large annual variations (Figure 6). The CPUE varied annually (Figure 8).

4.1. Catch Declined Due to a Decrease in the Total Number of Fishing Operators

Sea urchin fisheries have been practiced for a long time with fluctuating catches. The maximum catch was recorded in 1982, followed by a steady decline and subsequent fluctuations (Figure 2).

The total number of fishing operators decreased after 1991 (Figure 7). Logbooks collected from all fishing operators who fished for sea urchins at the study sites from 2020 to 2022 were examined, and a significant correlation was observed between the catch and the total number of fishing operators in each fishing ground [6]. In this study, the catch and the total number of fishing operators were tabulated annually, and a significantly strong positive correlation was observed for Anto and Saki (p < 0.05; Figure 9, Table 1). This indicates that a decline in the total number of fishing operators caused a decrease in catch.

4.2. Reason for the Decrease in the Number of Fishing Days

Fishermen voluntarily control their fishing efforts at the branch level of the Oshima Fisheries Cooperative [6]. Interviews with sea urchin fishermen confirmed that this system has been in operation since before 2020. The annual variation in the number of fishing days indicated that they were adjusted at the branch level for each fishing season (year) according to the natural stock abundance of sea urchins.

The Oshima Fisheries Cooperative has the exclusive right to fish for sea urchins at the study sites, and management is performed by each branch of the cooperative. If the number of fishing operators involved in the sea urchin fishery in each branch decreases, the available fishing ground area per operator increases. The daily fishing time for sea urchins is estimated to be approximately 7:00–10:00 [6], and it is reasonable for each branch to decide to increase the number of fishing days to increase the catch when natural stocks are abundant. However, the number of fishing days decreased annually.

Female divers perceived the abundance of the natural stocks of each fishing ground and stopped fishing when they caught a certain level relative to the abundance of the natural stock [6]. The number of fishing days was limited and decreased with annual fluctuations owing to the decline in natural stocks. Consequently, the total number of fishing operators declined, leading to decreased catches.

4.3. The CPUE Remains Low Despite a Reduced Number of Fishing Operators

If fishing grounds had abundant stocks, the CPUE would be high with few fishing operators. However, the CPUE was low, with few fishing operators in Anto, and it was not related to the total number of fishing operators in Saki (Figure 10).

As fishermen quantify the abundance of natural stocks through fishing, annual abundance cannot be determined without actual fishing. For example, even if the stock is poor and fishing is stopped immediately after the season, the CPUE for that year will be low because the divers fish for the sea urchin as usual immediately after the fishing season opens. Probably due to poor stocks in Anto over many years, the CPUE was low, with few fishing operators. Focusing on the years with fewer than 100 fishing operators in Saki, the CPUE varied significantly yearly (Figure 10). Therefore, it is plausible that the lack of a significant correlation between the CPUE and the total number of fishing operators in Saki can be attributable to substantial variability and/or fewer years of poor stocks in the fishing grounds compared with those of Anto.

Data were available for only a few years for Kaji and Komegawaki, and the CPUE was lower with fewer fishing operators, indicating that there were years when the abundance of natural stocks was low.

4.4. Sustainability of Sea Urchin Fishery and Future Challenges

The results of this study show that the number of fishing operators and the number of fishing days spent in the sea urchin fishery decreased. The number of fishing days decreased owing to a decline in the abundance of natural stocks. Understanding the abundance of a target species in its habitat is crucial for managing fisheries, particularly rock-dwelling species such as sea urchins [25]. Their mobility is limited compared with that of finfish, making them highly vulnerable to overexploitation by unregulated fishing, which depletes natural stocks. The fishermen had been voluntarily managing their fishing efforts for a long time, which has enabled them to sustainably harvest sea urchins over a prolonged duration (Figure 2). The fishing season avoids the sea urchin spawning season [10], and the catchable size of the sea urchin is regulated with a test diameter of >2 cm [8]. Despite continuous efforts in terms of fishing and regulations, the catch has declined because of the decrease in natural stocks.

For species such as sea urchins, which are heterogeneously distributed in the fishing grounds, stock management at fine spatial scales is necessary [26]. In addition, the aggregated distribution of sea urchins within fishing grounds highlights the significance of stock management at the scale of fishing grounds, including patches, to promote effective stock management [26,27]. Female divers have sensibly perceived the abundance of natural stocks at fine scales through fishing and have voluntarily managed their fishing efforts to prevent excessive pressure on certain fishing grounds [6]. In this study, we demonstrated that this management has been in place since before 2020 and it shows a system of effective stock management already established in the study sites. To achieve sustainable sea urchin fisheries, identifying the factors that cause fluctuations in the abundance of natural stocks and updating methods in conjunction with fishery management are essential. The fluctuations in the natural stocks may be influenced by factors such as climate change, pollution, and habitat degradation. However, these factors cannot be scientifically verified at this time in the study sites. Further studies on the cause of fluctuations and decline in natural stocks are essential for achieving a sustainable sea urchin fishery.

The sea urchin, H. pulcherrimus, inhabits boulders and their interspaces in fishing grounds. Traditionally, boulders are added to fishing grounds to create habitats for sea urchins [9,14]. Fishermen catch sea urchins by overturning boulders on the fishing grounds. When fishing grounds are no longer used, the boulders on the seabed gradually sink to the bottom, and sand and other materials accumulate, filling the spaces between the boulders. Thus, the space for sea urchins to inhabit disappears. When numerous fishermen harvest sea urchins, they turn over boulders on the seabed over a wide area of fishing grounds, thus preventing boulders from sinking to the bottom over an extensive area. This may have resulted in the maintenance and creation of habitat for sea urchins, which may have increased the abundance of natural stocks in the fishing grounds. However, recent reduced fishing efforts have reduced the areas where boulders are overturned by fishing. This may cause a vicious cycle in which the reduced overturned area leads to a decrease in profitable fishing grounds. Consequently, the abundance of natural sea urchin stocks has diminished further. If this cycle exists at the study sites, establishing marine protected areas alone will not resolve the problem. According to this background data, local residents, including fishermen, turn over rocks on the seafloor to maintain the habitat in addition to their fishing activities in the study sites. Further studies are essential to evaluate the effect of these activities.

An increase in seawater temperature is likely to have a negative effect on the habitat. The mean seawater temperature in the southwestern Sea of Japan, where the study sites are located, has increased by 1.51 °C over the past 100 years [28]. Additionally, the mean air temperature has increased by 1.35 °C over the past 100 years in Japan [29], which has contributed to an increase in seawater temperature in the habitat, as these habitats are located in shallow waters that are highly susceptible to increasing air temperatures. As mentioned above, rock-dwelling species such as sea urchins have limited mobility compared to finfish and cannot move out of their habitats when the environment becomes unfavorable. Therefore, the abundance of natural stocks is highly susceptible to changes in habitat.

This study show the trends in actual fishery operations in a sustainable dive fishery with continuous regulation and fishing efforts. However, focusing solely on regulations and fishing efforts does not lead to sustainable fishing. As stated above, fishermen have controlled their fishing efforts for a long time at fine spatial scales through voluntary fishery management. To effectively utilize this mechanism and enhance the effectiveness of stock management, the factors that drive stock fluctuations and declines must be surveyed continuously and the findings must be shared with the fisheries practicing voluntary management. Additionally, whether the current regulatory framework, such as catchable size and fishing season, contributes to improving stock management must be reassessed. The key to achieving sustainable sea urchin fisheries is clarifying the biological and ecological characteristics of sea urchins and updating fishing management methods in collaboration with fishermen and researchers. Ongoing efforts are necessary because environmental changes, such as rising air and seawater temperatures, are expected to persist in the future.

4.5. Limitations

This study has certain limitations. First, I did not consider weather conditions during the fishing season, even though they influenced decisions to conduct sea urchin fishing. The fishing season in the study area is typically calm, as it falls immediately after the rainy season and before the typhoon season, making it a suitable period for dive fishing. Therefore, I concluded that weather conditions did not influence the observed decreasing trend in fishing days. Second, I did not account for environmental and scale differences among fishing grounds, where each branch has exclusive rights to fish. Although the CPUE trends can be compared across branches, direct comparisons are not recommended.