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Assessing Key Fish Stocks of Seychelles’ Artisanal Trap and Line Fishery

2021

Summary

This assessment analyzed catch and effort data collected over 1990-2019 to determine the stock status of key artisanal fisheries in Seychelles. Fisheries included traps deployed by outboard vessels, which targeted reef-associated species in nearshore habitats, and handlines deployed by outboard and whaler-type vessels (lavenir, lekonomi, schooner, whaler), which targeted demersal and semi-pelagic species across the Mahé Plateau. These fisheries provide between 4,500 and 8,000 tonnes of catch per year, most of which is consumed in Seychelles, as well as high-economic value species (e.g. Lutjanus sebae) that are exported to international markets. Previous assessments have indicated that most of Seychelles’ artisanal fisheries are overexploited, with catch rates and total landed catch decreasing as fishing effort has increased.

Catch rates in trap fisheries increased steadily from 1990-2019, coinciding with a moderate increase in total fishing effort. Catch rates of species groups, however, declined over time, suggesting individual species have declined in biomass, or are more spatially variable. Traps primarily targeted either cordonier (Siganidae sp., the 4th most landed species group across all artisanal fisheries) or a mixed group of reef-associated herbivores and invertivores. Total catch and effort data indicate that cordonier are currently being fished below sustainable limits, and have not experienced overfishing in Seychelles, likely due to their fast population growth rate and thus high resilience to fishing pressure. In contrast, the mixed reef fish group was above sustainable limits, though it is difficult to estimate fishing reference points for this group, which contains over 30 species with varying life history traits. The available biomass of these species has also changed as a result of mass coral mortality throughout the inner Seychelles, likely contributing to both increases and decreases in catch rates.

Handline fisheries comprised the majority of landed catch from artisanal vessels in Seychelles, mostly carangues (Carangidae species), jobfish, and bourgeois (L. sebae) landed by whaler-type vessels on the Mahé Plateau. Across all vessel types, the average handline catch rate declined between 2005-2019. Outboard vessels had the most widespread catch declines, with catch rates decreasing for 8 of the 12 targeted species groups, suggesting that populations at inshore fishing grounds have become depleted. In contrast, whaler-type vessels showed evidence of recovering catch rates for in carangues, jobfish and bourgeois, but consistent declines for other vieille and maconde (grouper species), capitaine (Lethrinidae species) and becune (Sphyraenidae species). Despite negative catch trends, analyses of total catch suggested that most whaler-type handline fisheries were operating at or above sustainable limits. These predictions were likely due to recent increases in total landed catch that suggest demersal and semi-pelagic populations have recovered from high fishing pressure over 2000-2010. Nevertheless, catch-based reference points should also be accompanied by fishery-independent observations, information on population structure, and fishing ground maps, which were not available. Ultimately, drivers of catch trends (recovery or decline) were not clear, while there was considerable uncertainty in catch based reference points. Fishing by whaler-type vessels accounted for the majority of handline catches, and thus regulation of these fleets will likely help to protect long-term artisanal catches in Seychelles.

Over 2000-2016, fishing effort remained constant for trap fisheries (number of traps deployed), whereas outboard handline effort has increased from 2012-2016 and whaler-type vessel effort has decreased since 2000. Increased biomass of trap target species has likely sustained increases in total trap catch, though changes to catch composition should be further investigated. For handlines, reduction of fishing pressure may have enabled key stocks to recover, resulting in recent increases in total catch. However, these species vary considerably in life history and in fishing selectivity, and it is unlikely that all offshore stocks have recovered to similar levels.

Seychelles’ artisanal fisheries do not presently have harvest controls. Given the history of overexploitation and key economic and food security importance of these fisheries, the report recommends investigation of harvest controls and/or key habitat protection for four key stocks: carangues (handlines), bourgeois (handline), jobfish (handline), and cordonier (trap). These four groups are represented by one or more species with similar life history, are major contributors to total catch, and represent the most frequently caught species in three fisheries types (handline-pelagic, handline-demersal, trap-coastal). Harvest controls must be designed with consideration of the species’ life history and fishery dynamics, for example using size limits to minimize risk of growth overfishing of semi-pelagics, and protecting key habitats for aggregating species such as L. sebae and Siganus sutor. Routine biological monitoring of these species will be necessary to inform appropriate management measures (e.g. fishery-independent surveys; size and age estimates; spatial analysis of fishing grounds, aggregations and spawning habitat).

Finally, the Seychelles Fishing Authority’s catch assessment survey has provided a rich source of information on catch and effort across diverse and productive fishing fleets. These data will become increasingly important for gauging the effectiveness of harvest control rules, and minimizing risk of overfishing. It is recommended to fund additional fisheries science expertise within SFA, specifically to analyse catch data and help to transition the catch database into a format for rapid, real-time stock assessments.