Monitoring fisheries in data-limited situations A case study of the artisanal reef fisheries of Eritrea Promotor Prof. Dr. Johan A.J. Verreth Hoogleraar in de Aquacultuur en Visserij Wageningen Universiteit Co-promotor Dr. Ir. Leopold A.J. Nagelkerke Universitair Docent, leerstoelgroep Aquacultuur en Visserij Wageningen Universiteit Promotiecommissie Prof. Dr. M. Scheffer Wageningen Universiteit Prof. Dr. A.D. Rijnsdorp Rijksuniversiteit Groningen Imares, Wageningen Universiteit en Research Centrum Dr. Ir. J.A.E. van Oostenbrugge LEI, Wageningen Universiteit en Research Centrum Dr. I. Nagelkerken Radboud Universiteit Nijmegen Dit onderzoek is uitgevoerd binnen de onderzoekschool Wageningen Institute of Animal Sciences (WIAS) Monitoring fisheries in data-limited situations A case study of the artisanal reef fisheries of Eritrea Iyob Tsehaye Proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit, Prof. dr. M.J. Kropff, in het openbaar te verdedigen op dinsdag 13 november 2007 des namiddags te half twee in de Aula Tsehaye, I., 2007 Monitoring fisheries in data-limited situations: A case study of the artisanal reef fisheries of Eritrea PhD Thesis, Wageningen University, the Netherlands With summary in English, Dutch and Tigrigna ISBN: 978-90-8504-773-5 Dedicated to my parents: Tsehaye Weldemichael and Azieb Ghebrelul Abstract 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 Abstract 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. Contents Chapter 1 General introduction 1 Chapter 2 Rapid shifts in catch composition in the artisanal Red Sea reef fisheries of Eritrea 19 Chapter 3 Changes in catch composition in the multispecies fisheries of Eritrea in relation to biological and socio-economic factors 49 Chapter 4 Improving the effectiveness of monitoring programs in multispecies fisheries through aggregation of catch data into ecological categories 77 Chapter 5 Exploring optimal fishing scenarios for the multispecies artisanal fisheries of Eritrea using a trophic model 105 Chapter 6 General discussion 141 Summary in English, Dutch and Tigrigna 163 Acknowledgements 177 Training and supervision plan 179 Curriculum vitae 180 List of publications 181 CHAPTER 1 General introduction Chapter 1 1. Importance and management of reef fisheries Reef fisheries are important socially, nutritionally and economically, providing a direct source of income and food for hundreds of thousands of coastal communities (Sadovy, 2005). These fisheries are particularly important in developing countries, where coral reef ecosystems support an estimated annual yield of approximately 6 million tonnes per year (Munro, 1996; Allison and Ellis, 2001). Reef fisheries are inherently of multispecies nature, with any given type of gear capturing a wide range of species. The harvest from an intensively exploited coral reef ecosystem typically includes hundreds of species of fishes, although fewer than 20 species make up the bulk of harvests in most fisheries (Jennings and Polunin, 1996b; Kuster et al., 2005). Whereas some reef fisheries particularly target higher- value species such as groupers (Serranidae), snappers (Lutjanidae) and emperors (Lethrinidae), most reef fisheries of developing countries are non-selective, targeting every species of fish unless they are poisonous (Munro, 1996). As a result, reef fisheries are considered less wasteful, and hence more efficient, supporting more livelihoods with a given harvest than industrial fisheries (Sadovy, 2005). Compared with other fisheries, coral reef fisheries are nevertheless under-monitored and under-managed (Sadovy, 2005). There are several reasons why management is not so well developed in most reef fisheries. First, coral reef fisheries are mostly considered of low economic value at national and global levels, undermining the need for their monitoring and management (Kronen, 2004). Second, artisanal fishing techniques in most reef fisheries are perceived to be environmentally benign – although there is increasing awareness to the contrary (Sadovy, 2005) – with little impact on coral reef ecosystems (Hawkins and Roberts, 2003). In addition, fishing pressure is usually considered too low to have considerable effect on fish stocks. In addition, most reef fisheries are located in developing countries, where there are limited resources for their management. As a result, little focus has been given to the management of reef fisheries thus far, with interest being more on their development (Hawkins and Roberts, 2003). However, contrary to the general perception that marine ecosystems are hardly threatened by artisanal fishing, coral reef fisheries have widely come under pressure in the last few decades due mainly to artisanal fisheries (Hawkins and Roberts, 2003). Growing coastal populations, economic developments, improvements in fishing technology, rising living standards and globalization of trade have also exacerbated the pressure on many coral reef ecosystems (Dalzell, 1998; Russ, 2002). Accordingly, impacts of fishing have been documented for a 2 General introduction variety of tropical reef fisheries (e.g. Ruttenberg, 2001; Hawkins and Roberts, 2003). In the last 10–20 years, increasing fishing pressure resulted in marked declines in several fish