Mind the Depth: The Vertical Dimension of a Small‐Scale Coastal Fishery Shapes Selection on Species, Size, and Sex in Wrasses

Abstract

Small-scale fisheries (SSFs) tend to target shallow waters, but the depth distributions of coastal fish can vary depending on species, size, and sex. This creates a scope for a form of fishing selectivity that has received limited attention but can have considerable implications for monitoring and management of these fisheries. We conducted a case study on the Norwegian wrasse fishery, a developing SSF in which multiple species are caught in shallow waters (mean depth = 4.5 m) to be used as cleaner fish in aquaculture. Several of these wrasses have life histories and behaviors that are sensitive to selective fishing mortality, such as sexual size dimorphism, paternal care, and sex change. An experimental fishery was undertaken over three sampling periods in 2018. Data on catch, length, and sex of wrasses across a depth gradient (0-18 m) were collected and analyzed. We found that depth distributions were species specific and the vertical overlap with the fishery was high for Corkwing Wrasse Symphodus melops and Ballan Wrasse Labrus bergylta, which were most abundant at depths less than 5 m. Three other wrasse species had invariant or increasing abundance with depth and were therefore less likely to be negatively impacted by this fishery. Body size was positively correlated with depth for these wrasses, and sex ratio became more male biased for the Corkwing Wrasse, the only species that could be visually sexed. This study demonstrates that depth can have strong effects on fishing selectivity at multiple scales and that such knowledge is necessary to develop management strategies that

Movement patterns of temperate wrasses (Labridae) within a small marine protected area

“…The Norwegian wrasse fishery has expanded substantially during the last decade, which has resulted in reduced abundance and body size in exploited populations of goldsinny Ctenolabrus rupestris (L.) and corkwing wrasse Symphodus melops (L.) (Halvorsen et al ., 2017a,b). These two species make up ~90% of reported catch (Halvorsen et al ., 2020). The rock cook Centrolabrus exoletus (L.) is also frequently caught, but is typically discarded as bycatch.…”

“…These observations on wrasse movement are relevant for another potential challenge in this fishery; the survival of smaller wrasses discarded as bycatch (Halvorsen et al ., 2017b; Skiftesvik et al ., 2014). In Norway, fishers are obligated to return any bycatch immediately at sea close to shore, although not necessarily at the site of capture (Halvorsen et al ., 2020). The extent of this practice has not yet been assessed, but we highlight the potential of local negative consequences, such as higher predation risk for individuals that attempt to home or, in case of settlement in a new location, of increased competition for territories or food.…”

“…The fishery for wrasse is selective by species, size and sex, which increases the risk of overfishing (Halvorsen et al, 2016(Halvorsen et al, , 2020Kindsvater et al, 2020). Depletion of wrasses can have wider ecological consequences through top-down effects on smaller grazers and molluscs or via bottom-up effects on larger piscivores, since wrasses constitute an important part of the diet of seabirds and gadoid fishes (Dehnhard et al, 2021;Kraufvelin et al, 2020;Östman et al, 2016).…”

Restoration of Abundance and Dynamics of Coastal Fish and Lobster Within Northern Marine Protected Areas Across Two Decades

“…Goldsinny wrasse (Ctenolabrus rupestris) and corkwing wrasse (Symphodus melops) are key mesopredators on rocky reef habitats and used as cleaner fish in salmonid aquaculture to control sea-lice infestations (Gjøsaeter, 2002;Halvorsen et al, 2020). The salmonid aquaculture industry demand for wild caught wrasse increased considerably from 2010, when salmon lice developed resistance to various delousing chemicals, and the 1) are in light pink, while the no-take zone is shown in yellow.…”

Goldsinny wrasse (Ctenolabrus rupestris) have a sex‐dependent magnetic compass for maintaining site fidelity

“…As a result, wild-caught wrasse have been used in salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) aquaculture as cleaner fish to reduce infestations of the copepod ectoparasite Lepeophtheirus salmonis (Costello, 2009;Costello & Bjordal, 1990). The demand for wild-caught cleaner fish has driven the development and expansion of a lucrative commercial fishery (Halvorsen et al, 2020;Skiftesvik et al, 2014). Among the wrasse species present in Norway, goldsinny wrasse (Ctenolabrus rupestris) is one of the most abundant and is widely used as a cleaner fish (Skiftesvik et al, 2015).…”