Abstract
This study assesses the effects of a trawling ban on the diet and trophodynamics of the hake Merluccius merluccius by comparing stomach contents and stable isotopes (δ 15 N and δ 13 C) in two trawled gulfs and one untrawled gulf in northern Sicily (western Mediterranean). Comparisons were made for three size classes of hake encompassing 60 to 410 mm total length. Fish were collected from 50 to 200 m depth on muddy bottoms. The diets of hake of small and medium size were similar overall but more selective in the untrawled gulf. Greater differences were detected between the diets of larger specimens from trawled and untrawled areas. In the untrawled gulf large hake mainly preyed on clupeoid fish, while in the trawled gulfs other fish prey were found in the stomach contents. δ 15 N values of hake did not vary significantly between trawled and untrawled areas, while there was a clear effect of size, with larger individuals being significantly more enriched than juveniles. Conversely, δ 13 C values were generally more depleted for individuals collected in the untrawled area, suggesting a more pelagic source of carbon. The results from the mixing model agree fairly well with the known feeding habits found for each size class in each area.
Highlights
Trawling has many effects on marine ecosystems, including reduction of fish abundance and mean body size, decrease in diversity (Greenstreet and Hall 1996, Bianchi et al 2000) and physical habitat damage (Jennings and Kaiser 1998)
In the untrawled gulf large hake mainly preyed on clupeoid fish, while in the trawled gulfs other fish prey were found in the stomach contents. d15N values of hake did not vary significantly between trawled and untrawled areas, while there was a clear effect of size, with larger individuals being significantly more enriched than juveniles
The diet of class I hake was dominated by euphausiids in the three areas, with a higher amount in GTERM (70.63%N, 53.72%index of relative importance (IRI)) than in GSANT (62.94%N, 46.91%IRI) and Gulfs of Castellammare (GCAST) (58.14%N, 57.88%IRI) (Fig. 3A)
Summary
Trawling has many effects on marine ecosystems, including reduction of fish abundance and mean body size, decrease in diversity (Greenstreet and Hall 1996, Bianchi et al 2000) and physical habitat damage (Jennings and Kaiser 1998). A mix of direct and indirect effects is often observed at the food web level, the main one being a decrease in the mean trophic level as a consequence of the removal of larger individuals, which are generally predators (Bianchi et al 2000, Pauly et al 1998). Changes in the diet of fish can occur because of extra supply of organic matter from organisms killed or injured by the trawl gear and from discards that become food for opportunistic species (Kaiser and Spencer 1994). A significant change in feeding habits was observed in juvenile plaice Pleuronectes platessa because of bottom disturbance that increased the abundance of small infaunal invertebrates (Hiddink et al 2008)
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