Abstract
Demersal fish landings from the Firth of Clyde peaked in 1973, then declined rapidly until the targeted demersal fishery ceased in 2005. The abundance of large fish decreased during this period, and their numbers have not recovered since 2005. We aim to determine whether changing growth rates have contributed to the decline in the abundance of large fish. Bottom trawl survey data from 1980 to 2012 was used to calculate the annual mean length-at-age and time series of von Bertalanffy growth parameters of five demersal species; cod, haddock, whiting, Norway pout and saithe. Two regions were considered: the Firth of Clyde and the neighbouring seas west of Scotland (the western shelf). There have been substantial decreases in the lengths of most age groups of Clyde haddock and whiting due to declines in both asymptotic length and von Bertalanffy growth rate. Lengths-at-age have also declined in western shelf populations, but at markedly slower rates than within the Clyde. Trends in temperature and year class strength tended to contribute little to changes in the growth parameters, so declines in length-at-age have been largely due to other factors. Fishing intensity is greater in the Clyde than western shelf, and the size selectivity of the fisheries differ as more Clyde vessels use Nephrops trawling gear. Since trends in growth were also more extreme in the Clyde, it appears as though size-selective fishing may have caused reductions in the lengths of these fish. If the changes in growth are partially due to fishing induced evolution then it may take many generations for the changes to reverse.
Highlights
The Firth of Clyde is a large semi-enclosed body of water in the south-west coast of Scotland that has a long history of fishing
Length-at-age has declined in many distinct fish stocks around the world (Pope and Knights, 1982; Swain et al, 2007; Cheung et al, 2012; Audzijonyte et al, 2013; Baudron et al, 2014), and this has been observed in demersal populations within the Firth of Clyde as well as throughout the Scottish western shelf sea
The rapid declines in the mean length of Clyde haddock in age groups one and above were due to significant declines in L∞ and k (Table 4); the growth rate decreased steadily throughout the time series, whereas the decrease in L∞ occurred during 1998–2004 (Fig. 6)
Summary
The Firth of Clyde is a large semi-enclosed body of water in the south-west coast of Scotland that has a long history of fishing. Vessels over eight tonnes were prohibited from trawling in the Clyde from 1889 to 1962 in order to protect dwindling fish stocks, herring (Thurstan and Roberts, 2010). The declining seine net fishery for herring lead to the ban on trawling being lifted in 1962 to allow for the exploitation of demersal fish, Nephrops and scallops in areas further than three miles from the coast. Landings of demersal fish increased rapidly, reaching a maximum in 1973 before falling into decline (Hislop, 1986). Trawling within three miles of the coast was permitted from 1984 in an attempt to sustain the high catch levels. Responding to diminishing demersal fish catches, increasing numbers of fishing vessels began to directly target Nephrops during the 1990s. Since 2005 landings of demersal fish have come primarily from the Nephrops fishery bycatch
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