The effects of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems

“The impact of fishing on chondrichthyan stocks around the world is currently the focus of considerable international concern. Most chondrichthyan populations are of low productivity relative to teleost fishes, a consequence of their different life-history strategies. This is reflected in the poor record of sustainability of target shark fisheries.
Most sharks and some batoids are predators at, or near, the top of marine food webs. The effects of fishing are examined at the single-species level and through trophic interactions. We summarize the status of chondrichthyan fisheries from around the world. Some 50% of the estimated global catch of chondrichthyans is taken as by-catch, does not appear in official fishery statistics, and is almost totally unmanaged. When taken as by-catch, they are often subjected to high fishing mortality directed at teleost target species. Consequently, some skates, sawfish, and deep-water dogfish have been virtually extirpated from large regions. Some chondrichthyans are more resilient to fishing and we examine predictions on the vulnerability of different species based on their life-history and population parameters. At the species level, fishing may alter size structure and population parameters in response to changes in species abundance. We review the evidence for such density-dependent change. Fishing can affect trophic interactions and we examine cases of apparent species replacement and shifts in community composition. Sharks and rays learn to associate trawlers with food and feeding on discards may increase their populations.”

From the abstract of the study, which I found very interesting. Here’s some more stuff from the paper:

“Large-scale exploitation has led to changes in fish community structure. Fishers tend to remove the largest species first and then work their way down the food chain catching smaller species (Pauly et al., 1998). Consequently, changes in species composition of fished communities may be expected, with small, fastergrowing, and earlier-maturing species predominating. Small species may also be less desirable on the market, and may therefore be subjected to lower fishing mortality (Jennings and Kaiser, 1998; Jennings et al., 1999b). Within the chondrichthyans, the examples for skates discussed above reveal a broadly similar pattern. Similar patterns have also been reported in shark communities: as larger sharks were depleted smaller species proliferated (van der Elst, 1979). The general paradigm is that larger species decline while smaller species predominate.
There have also been declines in diversity associated with increasing fishing pressures, particularly in large predatory taxa (Jennings and Kaiser, 1998). Chondrichthyans tend to be high in the food web (Cortes, 1999) and, due to their greater vulnerability (relative to teleosts), are likely to be the first to decline from fishing. Rogers et al. (1999) suggested that fishing, through the differential vulnerability of elasmobranchs relative to teleosts, is responsible for major variations in fish diversity in the North-east Atlantic. […]

Discards from fisheries affect the amount of food available to scavengers and thus may be expected to have an effect on certain components of the ecosystem. Although some studies conclude that Australian prawn trawling had few significant, long-term impacts (Kennelly, 1995), about 95% of the by-catch in the Northern Prawn fishery is discarded, and most of it is dead (Wassenberg and Hill, 1989; Hill and Wassenberg, 1990). About half of the discards float and are scavenged by birds, dolphins, and sharks. The other half sinks and is preyed upon by sharks in mid-water and teleosts, sharks, and crustaceans on the bottom. […]

The predictions of the Venezuelan shelf ecosystem model under a mixed control assumption show that shark depletion could lead to strong and unforeseen changes in the abundances of many species (Fig. 4). According to the model, these changes would be permanent as long as shark populations remain depressed. Surprisingly, not all species whose abundances increased greatly are major prey of sharks. In fact, the species undergoing the greatest relative increases in abundance (croakers, snappers/groupers, grunts, catfish, and other demersals) are all minor components in the diet of the small triakid sharks, suggesting that shark depletion propagates through the food web in a complex way. Some changes are virtually demographic explosions of up to two and a half times the original biomass (i.e. croakers). Conversely, two of the major prey items of the sharks did not increase much in abundance; they even decreased (carangids and small pelagics). Squid and benthic producers, two groups not part of the diet, suffered abundance decreases of about 10% and 15%, respectively. Clearly, the outcomes are not as predictable as one might expect. […]

Chondrichthyans, by nature of their K-selected lifehistory strategies and high position in trophic food webs, are more likely to be affected by intense fishing activity than most teleosts. The group may in fact be indicators of fishing pressure. There is sufficient evidence from the history of fisheries around the world, both targeting these fishes and taking them as by-catch, of major declines in population size. For some groups, particularly certain skate species and sawfishes, there is mounting evidence suggesting that local if not global extinction is a distinct possibility. This problem is especially acute for species with restricted distributions. The massive and uncontrolled catch of chondrichthyans in the Indo-West Pacific, coupled with the higher diversity and rates of endemism in this region, are cause for major concern. There is increasing evidence that indirect effects of fishing are affecting the composition and diversity of chondrichthyan and total fish assemblages through trophic interactions. Differential vulnerability to fishing exists among sharks and rays and large, late maturing species appear to be most vulnerable. This has caused changes in the community through competitive release, although there is little evidence for species replacement. There is good evidence that selective fishing mortality can lead to changes in growth and juvenile survival for both sharks and batoids, leading to changes in population dynamics. However, the effects of removing large numbers of these top predators on the marine ecosystem are still largely unknown. Attention needs to be focused on this poorly studied group of fishes, particularly in the ecosystem context in terms of understanding trophic interactions.”

April 4, 2012 - Posted by | biology, evolution, studies

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