Paper: McClanahan, T.R.; Abunge, C.A. Catch rates and income are associated with fisheries management restrictions and not an environmental disturbance, in a heavily exploited tropical fishery. Marine Ecology Progress Series 513: 201-210, 2014. doi: 10.3354/meps10925
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Why we care
The scope of environmental problems can be daunting and sometimes hearing scientific news can be depressing. Corals are bleaching, climate change is threatening crop and water supplies, and fish stocks are projected to decrease in the future. However, it’s important to remember that without the sometimes depressing information, we can’t work towards effective solutions to these problems. And solutions do exist! For example, a northern state in Germany last year met 120% of their energy needs using renewable energy. We can combat climate change and other seemingly insurmountable problems!
This paper compares the influence of fishing pressure and environmental disturbance (increasing water temperatures, in this case) on coastal fisheries catches in Kenya. We actually know very little about the impacts of tropical fisheries management, despite the fact that demand for these resources is increasing while human and environmental impacts are becoming more severe.
The authors examined data on water temperatures, abundance of corals and other stationary reef organisms, and the number of fish caught over time, until 2011, along the southern coast of Kenya. The 1997 – 1998 El Niño event caused widespread increased ocean temperatures that altered stationary organism composition and abundance on the seafloor, coincident with fisheries closures and data collection.
Stationary organism and fisheries catch data was available starting in 1995 or 1996 for seven sites along the coast of Kenya and starting in 2001 – 2009 in nine additional sites. Stationary organism data was pooled into calcifying organism (like coral) percent cover and non-calcifying (like algae) percent cover, since these two groups of organisms may differentially affect fish abundances. Fish abundances were quantified using fish weights. Fish prices were combined with abundances to compute fishing revenues for analysis. Only seafloor-associated fish that fed in coastal environments were included in analyses.
Analyses were intended to compare the influence of sea temperatures, seafloor cover, and management strategies (divided into five groups based on management style and intensity) on fish catches. The authors used economic models commonly used to test for relationships between a set of variables and product prices. Analyses were able to specifically test whether temperatures or the two classes of reef organisms affected fish catches more than fisheries management did. The authors also tested for statistical trends in fish catches to determine whether the composition of catches changed over time.
Sea surface temperatures were not statistically linked to fish catches. Seafloor coverage by calcifying and non-calcifying organisms was correlated with catch rates, but cover changes did not precede catch rates and so there was no support for the hypothesis that cover changes influenced catch rates. Increased sea surface temperatures associated with the El Niño event, however, did cause changes in seafloor coverage.
After the El Niño event, catch rates and fisheries income increased as a result of fishing gear restrictions. Fish sizes for the most commonly caught species also rose during the study period. Revenue increases could be an artifact of a shift in the composition of fish catches, but statistical analyses indicated that this was not the case.
Scientists have thought for some time that reductions in seafloor coverage by reef corals and other calcifying organisms affect reef fish abundance and composition, but that these trends are difficult to detect because there is a lag time of five or ten years before these changes occur. This dataset covers more than ten years and does not support this hypothesis, although the authors cite other studies that do support it.
The authors credit this difference in outcome to the specific characteristics of Kenyan fisheries. Large-bodied predators at the top of the food chain have largely been fished out of these waters, perhaps leaving behind more disturbance-tolerant species which are also profitable when caught. The species that are caught in Kenyan coastal fisheries may also be less dependent on coastal reefs with nearby seagrass and sandy habitats providing alternative food sources. Finally, it is possible that the El Niño event caused a less intense and shorter-lasting disturbance on Kenyan coral reefs than elsewhere in the world, which may have impacted fisheries elsewhere more severely.
Nevertheless, the authors stress that at least in Kenya’s case, the impact of fisheries management cannot be overlooked. Fish catches and fisherman revenues responded positively over time to management-induced restrictions more than they responded negatively to one of the most impactful climate events in recent history. This suggests that in the face of globally-caused environmental problems with significant consequences, properly implemented local measures can positively influence natural resource supplies. There is hope.
I just finished my graduate education in the Odum School of Ecology at the University of Georgia. I received my Ph.D. in Ecology in August 2014. My dissertation is all about the creatures that make the habitat for an ecosystem just by growing themselves. I’ve done my research in mangroves; trees that live at the edge of the ocean in the tropics. Before coming to UGA, I earned my B.S. in Biology from the University of North Carolina at Chapel Hill, where I worked on a variety of marine ecology projects.