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Population density and fishery impacts on the sea cucumber (Isostichopus fuscus) in the Galapagos marine reserve

M. Verónica Toral-Granda and Priscilla C. Martínez[22]


Charles Darwin Research Station, Santa Cruz, Galapagos

Abstract

Over the last decade, the sea cucumber Isostichopus fuscus has been the target of a continuous fishery in the Galapagos Islands. The increasing growth of this activity and its potential impacts prompted the Park Authorities to initiate a participatory management program to assess the status of the resource. From 1999 until 2003, density surveys of I. fuscus have been conducted before and after each fishing season by teams of fishers, naturalist guides, managers and scientists. Using a 100 m2 sweep circular transect an average of 900 m2 have been surveyed in specific sites of Fernandina, Isabela, Española, Floreana, Santa Cruz and San Cristóbal. A drastic decrease of density and size structure of I. fuscus has been observed after each fishing season, with population densities partly recovered between fishing periods. In Isabela and Fernandina, a single recruitment event was recorded in April 2000, which reached its peak in March/April 2001, which probably helped the ongoing fishery on those islands. No recruitment has been detected on any other island. Nonetheless, current adult and juvenile densities show that I. fuscus populations in the Galapagos Islands are severely depleted and unless there is another recruitment pulse, along with a complete ban on fishing activities these populations are in serious risk.

Keywords: Population dynamics, surveys, recruitment, participatory monitoring

Introduction

Sea cucumbers have been harvested for years in the coastal zones of temperate and tropical regions of the world, mainly throughout the Indo-Pacific, with temperate fisheries in the North Pacific and tropical fisheries in the Indian and Western Pacific islands (Conand and Sloan, 1989). The rapid decline of sea cucumber populations worldwide to support the beche-de-mer market (Conand, 2001) induced the beginning of this fishing activity in continental Ecuador in 1989 focusing on Isostichopus fuscus Ludwig 1875 (Carranza and Andrade, 1996). A few years later, after economical depletion of this species in mainland Ecuador (Camhi, 1995) this activity began in the Galapagos Islands in 1991 (Camhi, 1995;, De Paco et al., 1993; Powell and Gibbs, 1996). Although this fishery was expected to be mainly local (Powell and Gibbs, 1996), it brought a wake of illegal mainland fishers into the local community and caused a series of conflicts which led to the ban of this fishery in 1992 (De Paco et al., 1993). During the first legal fishery in 1994, the fishing activity focused in the Canal Bolívar (between the islands of Isabela and Fernandina) (Figure 1) due to greater abundance of I. fuscus in this region. However, after the reopening of the fishery in 1999, the fishing grounds extended to Isabela, Fernandina, Floreana, Española, San Cristóbal and Santa Cruz with the potential for expansion to the rest of the archipelago (Figure 1).

This species (Isostichopus fuscus) is the most common commercial species found in the Eastern Pacific (Maluf, 1988) and was once known as the most conspicuous invertebrate of the shallow littoral zone in the Galapagos Islands (Wellington, 1974). Its distribution includes Baja California, México to mainland Ecuador, including Galapagos, Socorro, Cocos and Revillagigedos islands (Deichmann, 1958; Maluf, 1991). It can be found from the coastal zone to 28-39 m depth (Deichmann, 1958; Maluf, 1991). However, this species was declared in risk of extinction in Mexico (Secretaria de Medio Ambiente, Recursos Naturales y Pesca, 2000) and has very low populations in mainland Ecuador.

Figure 1. Participatory monitoring sites in the Galapagos Marine Reserve since 1999. Only islands with dark circles (sites) have been monitored. The section in the frame is known as the Bolívar Channel.

Isostichopus fuscus is collected from the sea bottom by hand by hookah divers at depths between 1 and 35 m. The Galapagos National Park Service (GNPS) is the entity in charge of managing the fishery. This fishery has a Total Allowable Catch (TAC) which varies year to year (Murillo et al., 2003), minimum landing size of 20 cm for fresh individuals and 6 cm for dried individuals and no-take zones distributed in different islands. Although the sea cucumber fishery is the most important fishing activity in the Galapagos Islands (Murillo et al., 2002) there are still some areas of scientific interest that need to be covered in order to sustain the fishery. Most of the information available is on population density (Martínez, 1999; Toral-Granda et al., 2003), reproductive biology (Toral-Granda, 1996), growth and basic ecology, fishery information (Bustamante et al., 1999; Toral-Granda et al., 2000; Murillo et al., 2003). The fishery shows evidence of overexploitation (Murillo et al., 2002, 2003) with average catch per unit effort values decreasing with time [(i.e. from an average of 109 individuals diver-1 day-1 in 1999 to 82 individuals diver-1 day-1 in 2003 (Murillo et al., 2003)], increased claims of fishers for the opening of other islands to this activity and decrease in size of landed individuals.

Since the establishment of the Galapagos Marine Reserve (GMR) in 1998 through a participatory process, all major stakeholders, i.e. tourism, fishing, naturalist guides, science & conservation (Charles Darwin Research Station) and management (GNPS), have taken active part in the population monitoring of I. fuscus. Twice a year since 1999, participatory monitoring trips have taken place prior to and after the fishing season, aiming to determine the impacts of the fishing activity and potential recovery of this species during bans and closed zones.

This paper focuses on the results of the participatory monitoring censuses in Española, Fernandina, Floreana, Isabela, San Cristóbal and Santa Cruz from 1999, giving an up to date overview of I. fuscus in the GMR after five fishing seasons, with special focus on the impact of fisheries on this species, its recovery during fishing bans and closed zones and the implication of the current fishing practices on the well-being of this species.

Participatory management of the Galapagos Marine Reserve

In 1998, the Ecuadorian Government in response to both national and international pressure for a better management of the Galapagos Islands passed on the Special Law of Galapagos, which led to the creation of the GMR. The GMR management consists of a three-pole arrangement with the Participatory Management Board (PMB), the Interinstitutional Management Board (IMA) and the GNPS. The PMB members are based in the Archipelago and represent the direct users where any decisions concerning the management of the GMR is taken on a consensus based system. The IMA is on a ministerial level where all decisions taken in the PMB are approved by a voting system. The GNPS is the entity in charge of putting into effect all resolutions taken by both the PMB and the IMA (Altamirano et al., 2004).

The PMB comprises the participation of all direct stakeholders: artisanal fishing sector, naturalist guides, tourism sector, science, conservation & education (Charles Darwin Foundation) and the management authority (GNPS). Hence, when the Ecuadorian Government approved a sea cucumber fishing season in 1999, the need was recognised to integrate all stakeholders in the gathering and analysis of the scientific information in the GMR.

Twice a year, before (between January and April) and after (between June and August) each fishing season a team, comprised of fishers, managers, naturalist guides and scientists, have visited permanent monitoring sites located in Española, Fernandina, Floreana, Isabela, San Cristóbal and Santa Cruz islands (Figure 1) over an average of 10 working days. At each site (Figure 1) a minimum of 900 m2 were surveyed using a circular sweep transect specially developed for holothurians and in use in Galapagos since 1993 (Richmond and Martínez, 1993). Surveys were performed by a team of two divers using a rope of 5.64 m length, weighted at one end, which is used as a rotation point. While one diver holds the end of the rope and swims in circles, a second diver counts and collects all individuals found within the circle. The total area covered once the circle is completed is 100 m2 (Figure 2). At the surface, all collected individuals were measured to the nearest centimetre and then returned to the collection point. All scientific information was analysed and then presented to the PMB and IMA for further decisions on the status of I. fuscus in the GMR.

From 2001 onwards, a threshold value was set to determine whether a fishing season will take place on the GMR. The threshold value was set on 0.4 ind. m-2 of individuals > 22 cm total length based on the information gathered in 2000. For the 2001 fishing season, a population survey was carried out at all sites and despite that only one of the sites met the threshold value, the fishery was opened (Martínez et al., 2001). However, as from then, rotation of islands has been promoted in order to protect, to a certain extent, the well-being of I. fuscus. For further details on the management scheme on / fuscus see Altamirano et al. (2004).

In order to evaluate the effect of the GMR coastal zoning (approved in 2000) and permanent monitoring on the population of / fuscus, a subset of 17 sites that have been monitored 8 out of 10 times was used (Table 1). These sites belong to the three subzones of protection: subzone of Comparison and Protection (2.1), subzone of Conservation and Non-Extractive Uses (2.2) and subzone of Conservation, Extractive and Non-Extractive Uses (2.3).

Figure 2. Circular transect used to gather population density and size structure of I. fuscus in Galapagos. Drawing: Fernando Pinillo.

Table 1. Sites analysed to evaluate the impact of No-Take Zones (NTZs) in the Galapagos Marine Reserve.

Subzone 2.1[23]

Subzone 2.2[24]

Subzone 2.3[25]

Punta Cevallos (Española)

Bahía Elizabeth (Isabela)

Chorros de Agua Dulce (San Cristóbal)


Las Cuevas (Floreana)

Veinte Varas (San Cristóbal)


Punta Mangle Sur (Fernandina)

Puerto Bravo (Isabela)


Punta Espinosa Sur (Fernandina)

Punta Mangle Norte (Fernandina)


Punta Espinosa Norte (Fernandina)

Bahía Urbina-Poza de los Chinos (Isabela)



La Montura (Floreana)



Manzanillo (Española)



Botella Grande (Floreana)



Patrullera (Isabela)



Las Negritas (San Cristóbal) Las Palmas (Santa Cruz)

Results

Between 1999 and 2003, a total of 69 sites have been monitored throughout the islands with over 325 000 m2 surveyed in five years. The number of sites has varied between years, with a subset of 17 sites having 80 % of the visits. As from 2000, there was a drastic impact of the fishing seasons on populations ofI. fuscus around the archipelago, with a recovery during the ban (Figure 3). Current density values are 39 % lower than those found in the survey before the 1999 fishing season and 85 % lower than those registered after the highest peak registered in the pre-fishery survey in 2001. Due to financial constraints, in 2001 there was no post-fishery population survey in the southern islands (Española, Floreana and San Cristóbal) which explains the low variation between the pre- and post-fishery surveys.

In addition to the great fluctuation in the abundance of I. fuscus, the total abundance of individuals > 20 cm has also changed during the study period, with the lowest values mostly in the post fishery surveys (Figure 4). However, the biggest decrease in the abundance of such individuals was found in the survey after the 2002 fishing season and with a further decrease in the 2003 post-fishery survey. The highest density value was recorded in the pre-fishery survey of 2001 (Figure 4). The abundance of I. fuscus < 20 cm was low in 1999, with a sudden increase in the pre-fishery survey in 2000 followed by a drastic drop after the fishing season and a recovery in 2001, which remained relatively constant until the pre-fishery survey in 2002. After this period, there is a notable decline in the population for both, larger and smaller individuals, and in 2003 density values were lower than in any previous surveys (Figure 4).

Figure 3. Average density (ind. m-2) of I. fuscus at all sites monitored in the before and after fishery surveys between 1999 and 2003.

Figure 4. Density of I. fuscus below and above 20 cm TL (Total Length) in all sites monitored since 1999.

The greatest variation of total average density was observed in Fernandina and Isabela, where the values were almost an order of magnitude different than those observed elsewhere (Figure 5). The drastic recovery in the 2000 and 2001 seasons may be due to a recruitment pulse that was observed then, which lasted until the beginning of 2002 when it started to diminish. This figure also shows the lack of recruitment pulses in the other islands, where in most cases the average density is very low and does not show any sign of recovery (Figure 5).

Figure 5. Average density of I. fuscus in the six islands surveyed since 1999.

From 1993 to date, five sites in the western part of the archipelago (between Western Isabela and Fernandina) have been monitored - Punta Espinosa Norte, Punta Espinosa Sur, Punta Mangle Norte and Punta Mangle Sur in Fernandina and Bahía Elizabeth in Isabela. It is as a result of this monitoring effort that the only recruitment event ever observed in the GMR could be detected (Figure 6). From April 1993 until November 1999, juvenile density (< 16 cm)[26] is almost non-existent (Figure 6), however, as from April 2000 the value for this size class increases drastically, remaining fairly high in the pre-fishery surveys of 2000, 2001 and 2002 (April and December 2000, April 2001 and March 2002). Despite the minimum landing size (20 cm fresh length) it can be observed the substantial decrease of juvenile individuals after each fishing season (Figure 6).

Total numbers of adults (³ 16 cm TL - Total Length) have also fluctuated throughout the study period in these five sites. Despite the ban on fishing on this species in Galapagos from 1994 until 1999, the population density of I. fuscus in the Bolívar Channel has decreased throughout time, achieving very low values just before the observed recruitment event (Figure 6).

Figure 6. Population density of juveniles (< 16 cm Total Length) in five permanent monitoring sites located in the Bolívar Channel (between western Isabela and Fernandina): Punta Espinoza Norte, Punta Espinoza Sur, Punta Mangle Norte, Punta Mangle Sur in Fernandina andBahía Elizabeth en Isabela (De Paco et al., 1993; Richmond and Martínez, 1993; Martínez, 1999).

The coastal zoning in the GMR is yet to prove its benefits. Populations of I. fuscus within No-Take Zones (2.1 and 2.2 - see Table 1) still present a dwindling pattern (Figure 7). The impacts of the fishing seasons can be easily observed especially in subzone 2.2 and subzone 2.3 where the values decrease drastically after the fishing season. However, in Punta Cevallos (subzone 2.1) the population density tends to decrease with time (Figure 7).

Figure 7. Average density of I. fuscus in No-Take Zones (Punta Cevallos-2.1 and 2.2) and in fishing zones (subzone 2.3).

Discussion

Since the start of the sea cucumber fishery in 1991 a considerable amount of sea cucumbers have been harvested both legally (De Miras et al., 1996; Murillo et al., 2003) and illegally (De Miras et al., 1996; Martínez, 1999; Piu, 1998, 2000) which has yielded fluctuating populations. The constant reduction of I. fuscus in the GMR has lead to the fact that this species can no longer be considered the most conspicuous of the subtidal zone (Wellington, 1974) and its removal can yield changes in the ecosystem functioning due to its important role as a nutrient recycler and bioturbation agent (Bakus, 1973; Conand, 1993).

Sea cucumber fisheries worldwide have presented cycles in which the total catch decreases despite an increase in the fishing effort, which in turn leads to the overexploitation of the species and low economic returns (Aguilar-Ibarra and Ramírez-Soberón, 2002). This trend of exploitation has been recorded in various places, including México (Aguilar-Ibarra and Ramírez-Soberón, 2002), mainland Ecuador (Carranza and Andrade, 1996) and now in the Galapagos Islands, showing once more that this group of organisms is easy to collect and prone to overexploitation.

Legal fishing activity in the Galapagos Islands has led to the extraction of approximately 31 million individuals (De Miras et al., 1996, Murillo et al., 2003), with most of the catches from Western Isabela and Fernandina (Murillo et al., 2003). Catch values from other islands have decreased with time, with certain islands (i.e. Española) with very low values (Murillo et al., 2003), which prompted the closure of that island for the 2003 fishing season. From the fishery independent data[27], only those populations in western Isabela and Fernandina are still able to sustain fishing activities, however, it is only in this region that recruitment has been observed.

The total density of I. fuscus (> 20 cm TL) found during the post-fishing survey in 2003 are the lowest ever recorded in the GMR. Isostichopus fuscus is a broadcast spawner whose successful reproduction relies on the proximity of its conspecifics. Studies done elsewhere show that there is a 75 % reproductive success when the density of mature animals is between 5 and 8 ind. m-2 (Hamel and Mercier, 1996) but when the nearest neighbour is metres away, successful reproduction may go down to zero (Levitan et al., 1991). Fifty percent of the / fuscus population reaches sexual maturity between 21 and 23 cm TL (Toral-Granda, 1996; Martínez, 1999), however, with less than 10 % of the population within this size range the whole survival of this species may seem to be in peril. Current adult densities of I. fuscus in the south and central islands are quite low, which may mean that the 'Allee Effect' is likely reducing recruitment levels below those necessary to sustain both the fishery and the population (Shepherd et al., in revision). Additionally, according to Shepherd et al. (in revision) a minimun of 1.2 ind.m-2 is required for a 50 % fertilization success in / fuscus. Tegner and Dayton (1977) in studies of other species of echinoderms have proved that when there is not a high enough density of adults, recruitment may not be sufficient to compensate fishing mortality.

The recruitment pulse that was observed in 2000 and 2001 is coming to an end, showing the vulnerability of this population to overexploitation when there is not a known cycle of reproduction happening in the GMR. If an / fuscus population relies on external factors (i.e. El Niño and La Niña) to favour the successful reproduction and settlement (Murillo et al., 2002) of this species, then extra care must be taken in its management so as to avoid the collapse of this species and even its possible commercial extinction in the Ecuadorian territory.

Although the information on the importance of NTZs for the benefit of I. fuscus populations in the GMR is not clearly evident this could be attributed to the relatively short period of time of the NTZ's existence, thus it is too early to see recovery of the populations. Additionally, the limited patrolling activities due to financial constraints can mask the results. But another fact is that there is a certain level of ignorance among fishermen about the new zoning scheme of the GMR and poaching continues. However, it is important to continue this area of research as the benefits of NTZs for populations of both marine vertebrates and invertebrates have been proved elsewhere (Edgar and Barrett, 1996; Allison et al., 1998; Nowlis and Roberts, 1999; Ward et al., 2000). Despite the great efforts from the Galapagos National Park Service (GNPS) to control and enforce the NTZ, there is still a substantial amount of illegal fishery within the GMR (Altamirano and Aguiñaga, 2002). This lack of adherence to the existing legal framework of the GMR makes it difficult to obtain reliable data on of the effect of the NTZs. Yet, only the continuation of this type of study along with an increased enforcement by the GNPS will prove the possible benefits of all NTZs in the GMR.

The sea cucumber (I. fuscus) populations in the GMR are seriously endangered due to commercial exploitation. Only a comprehensive management plan that will involve the best available scientific information, together with a serious commitment of the fishing sector to follow the rules imposed in the plan and the continues control of the GMR authorities, may allow any commercial activities to be sustained over time. Further research is needed to clarify the role of density dependent factors, the 'Allee Effect', oceanographic parameters, amongst other topics, that could lead to a better understanding of the Isostichopus fuscus population dynamics and would improve our assessment and management of this important resource.

Acknowledgments

The United States Agency for International Development (USAID), the United Nations Development Program, the Inter-American Development Bank (1274 OC/EC), the Galapagos National Park Service and Artisanal Fishers Cooperatives Association provided funds for participatory surveys since 1999. Chantal Conand, John Ryder, Alessandro Lovatelli, Tom Poulsom and Eva Danulat kindly reviewed and improved the document.

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[22] Present address: Zoology Department, University ofMelbourne, Parkville, Victoria 3052, Australia.
[23] Only scientific use allowed.
[24] Only scientific and tourist use allowed.
[25] Fishing activities allowed in this subzone, as well as scientific and tourist use.
[26] According to Toral (1996) size at first maturity is 16 cm total length.
[27] Data gathered independently of the fishing season for this species. It normally implies information from the before and after fishing season.

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