Could our fisheries be more productive? Indirect negative effects of bottom trawl fisheries on fish condition

Abstract

Summary1. The nets used in bottom trawl fisheries cause mortality of benthic invertebrates and this can decrease the long-term availability of prey to exploited fish species by reducing the abundance of benthic invertebrates. This may have consequences for the sustainability of fisheries. 2. We assessed the impact of bottom trawling on the food availability of fish by comparing the condition of fish (as weight-at-length) in an area that had a steep commercial bottom-trawling gradient in the Irish Sea but otherwise homogeneous environmental conditions. 3. We found that the condition of the important commercial flatfish plaice Pleuronectes platessa was negatively related to trawling frequency, and this could be explained by a reduced production of the infaunal invertebrates they feed on. Density-dependent changes in competition over food could not explain this difference. No effect of trawling on the condition of the flatfish dab Limanda limanda was detected. Whiting Merlangius merlangus feeds primarily on fish, and therefore, no effect of bottom trawling on its condition was expected or detected. 4. This study therefore indicates that bottom trawl fisheries may have a negative effect on the condition of some of their target species, but not others, by reducing the abundance of their benthic prey. 5. Synthesis and application. Bottom trawls may indirectly affect the population size and growth rate of the target fish species and result in lower fishing yields. Such reductions in the yield and sustainability of fisheries are highly undesirable. The effects of bottom trawls may be mitigated by the modification of fishing gears or by minimizing the area of the seabed fished by bottom trawls.

Effect of intra‐specific competition, surface chlorophyll and fishing on spatial variation of gadoid's body condition

“…2) and thus juveniles and young adults could be benefitting from reduced competition as well as the proliferation of benthic prey species. On the contrary, the effect of fishing effort on body condition of recruits of P. blennoides was negative, which could be explained by a reduction of their main prey as v www.esajournals.org observed in the recruitment grounds of other areas (Hiddink et al 2011). Local effects of fishing effort on recruits of M. merluccius and M. poutassou present a consistent latitudinal gradient, with stations north and south of the Balearic Islands showing contrasting responses in body condition.…”

“…The impact of trawling disturbance on the marine environment has been widely investigated, evidencing mostly negative effects from populations to ecosystems properties (e.g., Jennings et al 2001, Kaiser et al 2002, Hiddink et al 2011, and references therein), including fish condition (e.g., Choi et al 2004, Hiddink et al 2011, Lloret et al 2014. Conversely, positive effects of low levels of trawling have been either hypothesized (e.g., Rijnsdorp et al 1991, Jennings et al 2001, Daan et al 2005 or proved (Hiddink et al 2008, Giacalone et al 2010, Walsh et al 2012, mostly due to removal of the larger specimens and leading to three non-exclusive processes: (1) lower predation rates, resulting in higher benthic prey availability, (2) reduced competition among benthic fauna over food and space, which leads to increases in the production of the small benthos that fish feed on, and (3) proliferation of smaller, more productive benthic species (''farming the sea'').…”

“…Total length was also included in the models as previous exploratory analysis showed a clear effect of length on body condition. Station was considered as random effect (Hiddink et al 2011).…”

“…The effect of surface chlorophyll a is often positive (Basilone et al 2006, Rueda et al 2014, though its influence is observed at different temporal lags (Hidalgo et al 2008, Rueda et al 2014). In addition, several authors described a negative effect of fishing on body condition (e.g., Choi et al 2004, Hiddink et al 2011, Lloret et al 2014, whereas others have detected positive effects at low levels of fishing pressure (Giacalone et al 2010) or absence of a statistical significant relationship Planes 2003, Giacalone et al 2010). These contrasting effects prove the complexity of such interactions and suggest the necessity of addressing the influence of the density-dependent, environmental and anthropogenic drivers on body condition in an integrative manner.…”

Evaluating the effect of fishery closures: Lessons learnt from the Plaice Box

“…There is also support for this theory from scientists (Hiddink et al, 2008) who show that certain levels of bottom trawling disturbance may enhance the production of food for plaice (small invertebrates), although a recent study suggested that fish condition was negatively related to trawling intensity (Hiddink et al, 2011). The fishermen argue that they are now actually an important part of the ecosystem itself working to enhance marine productivity.…”