Monitoring fisheries in data-limited situations : a case study of the artisanal reef fisheries of Eritrea

Tsehaye, I.W.


Elucidating trends in catch rate and composition is important to evaluate the impact of fishing on fish stocks, and thereby guide fisheries management action. Since major changes in fish community structure can take place even at the initial stages in the development of fisheries, the onset of reef fisheries in the Eritrean Red Sea provides a good opportunity to assess the effects of fishing on fish community structure in the early stages of fisheries. The artisanal reef fisheries of Eritrea were launched into a renewed development in 1991 after the end of a decades-long standstill, which was caused by the war for Eritrea’s independence. The main objective of this thesis was to evaluate the sustainability of existing fishing practices in the newly developing artisanal reef fisheries of Eritrea, while simultaneously exploring more effective ways of monitoring of fisheries and fish stocks. Analysis of official statistics from the artisanal fisheries showed that annual effort and annual catch increased upon the expansion of the fisheries, while the total catch rate remained unchanged over the years. However, disaggregation of catch rates into different taxonomic and ecological categories showed varying trends in catch rates of the different groups of fishes, resulting in a clear shift in catch composition over the years. These results are not unlikely given the supposedly large amounts of unreported catches, which are mostly taken illegally to Yemen. This is more so given the highly selective nature of species targeting strategies in the artisanal fisheries. These findings are also consistent with other studies suggesting that stocks of reef-associated demersal species are highly vulnerable to fishing and can decline even at the early stages of fisheries. Furthermore, the results from this study show that the impacts of fishing in the artisanal fisheries could even become more severe if market conditions improve leading to the activation of latent fishing capacity. That having been said, even though the changes in catch rate could be interpreted as reflecting changes in the underlying fish community structure, they could also be partly due to shifts in fish targeting strategies driven by socio-economic circumstances. The relationship between catch rate and effort in the artisanal fisheries could be elucidated more accurately by standardizing catch rates for systematic variations among fishing vessels, which would otherwise cause non-random variations in catch rate unrelated to abundance. The effectiveness of the monitoring program in these fisheries could also be improved through aggregation of catch data. The catch data from the artisanal fisheries could be aggregated into three distinct ecological categories, without considerable loss of information on trends in the catch rate of individual species. These categories included reef-associated demersals, open-water pelagics and near-reef pelagics. Aggregation of catch data into these ecological categories was found to improve the statistical power to detect trends. Moreover, besides improving statistical power, aggregate catch rates could provide better insight into community-level responses to exploitation. An Ecopath with Ecosim model was used to verify if the fishery-induced changes in yield and abundance of fish implied by official statistics are likely, given the existing levels of fishing intensity. This model was also used to explore optimal harvesting strategies for the artisanal fisheries by examining the ecosystem effects of fishing. A retrospective simulation of trends in catch rate (yield) using Ecosim showed that, given the existing fishing intensity as suggested by official statistics, a decline in yield or abundance of reef-associated demersals of the magnitude suggested by trends in catch rate are unlikely. The decline in yield observed in these fisheries could however be reproduced in an Ecosim simulation when the existing fishing level was assumed to be fivefold the official estimate, as suggested by anecdotal evidence on unreported catches. The simulation results also suggest that an optimal fishing strategy could be achieved in the artisanal fisheries through a slight reduction in the annual catch of reef-associated fishes and an increase in the capture of near-reef pelagics and large pelagics from the putative levels of fishing.