Year-class strength of fish stocks is often set during the larval stage, with fast growth being a favourable factor leading to strong recruitment. Following 30 years of poor recruitment, redfish (Sebastes sp.) in the Gulf of St. Lawrence (GSL) produced unprecedentedly strong year classes in 2011–2013. The relationship between larval diet and growth that potentially drove these successful recruitment events is unknown. Gut content and otolith microstructure of redfish larvae collected from the GSL in 1999 and 2000 were analysed in order to understand the relationship between recent feeding success and growth in larval redfish, and identify feeding behaviours that are associated with fast growth. Growth was positively correlated with larval body depth (BD), which was used as a morphological proxy for growth. Prey type and size were the best explanatory variables of larval BD after obligatory first-feeding. Larvae that consumed large naupliar stages of frequently consumed copepod taxa were deeper-bodied and grew more quickly than larvae that consumed redfish's preferred prey, Calanus finmarchicus eggs. Warming GSL waters have shifted the phenology of commonly consumed prey taxa earlier in the season, which may increase the overlap between redfish and naupliar prey that drive fast growth, survival, and potentially recruitment success.
Spatiotemporal overlap between fish larvae and their planktonic prey is an important source of recruitment variability. Over the past decade, one species of redfish, Sebastes mentella, from the Gulf of St. Lawrence (GSL) produced multiple strong cohorts following decades of low recruitment, which has generated strong interest in identifying potential drivers of larval survival. The present study provides the first detailed, multi-year assessment of larval redfish (Sebastes spp.) trophodynamics. Interannual variability in larval redfish diet composition and prey selectivity was assessed using high-resolution prey identification of larval gut contents and in situ prey fields. Eggs from the calanoid copepod Calanus finmarchicus represented the most frequently consumed prey in 3 of the 4 collection years, and contributed the largest proportion of carbon ingested by redfish larvae in all years. The high consumption of C. finmarchicus eggs by larvae, combined with evidence of positive selection for this taxon in some years, supports the hypothesis of a strong trophic link between larval redfish and a key calanoid copepod in the GSL ecosystem. Our results indicate that future efforts investigating GSL redfish recruitment processes should consider environment-driven variability in the reproductive phenology and abundance of C. finmarchicus.
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