Spatio-temporal models provide new insights on the biotic and abiotic drivers shaping Pacific Herring (Clupea pallasi) distribution

Abstract

Determining how fish respond to variation in biotic and abiotic conditions is a crucial prerequisite to forecasting changes in productivity and spatial distribution of fish stocks and designing sustainable marine resource management strategies. In the present study, we investigated the physical and biological drivers of the spatio-temporal dynamics of Pacific Herring (Clupea pallasi), which is a marine forage fish species important for commercial fisheries and First Nations in the northeastern Pacific. We fit multivariate spatio-temporal models to fisheries-independent trawl- and acoustics-based data collected off the West Coast of Vancouver Island (WCVI), Canada, during summers over the period 2006–2014. We evaluated the effects of the main ocean environmental drivers of WCVI lower trophic level productivity, including sea surface temperature (SST), chlorophyll a, fluorescence, salinity, oxygen, transmissivity and zooplankton density on variation in Pacific Herring biomass. Models were also used to measure spatio-temporal covariation with other pelagic, semi-pelagic, and bottom-associated fish species occurring off the WCVI to address potential competitive and predation interactions. Through application of these spatio-temporal models we found: (i) Pacific Herring biomass off the WCVI increased during 2006–2014; the highest Pacific Herring biomass was repeatedly found on the continental shelf (depths < 185 m) while low densities were usually observed along the shelf break (depths > 185 m), where Euphausiids, Pacific Hake, Sablefish and Arrowtooth Flounder were more abundant, which could reflect predation avoidance behaviour; (ii), the local biomass of Pacific Herring was related quadratically to the average SST in May; (iii) a positive covariation in spatio-temporal densities between Pacific Herring and its common zooplankton prey, supporting a potential bottom-up control hypothesis; (iv) a negative covariation in spatio-temporal densities between Pacific Herring and both Pacific Hake and Pacific Sardine, which could reflect predation and competitive interactions, respectively; and (v) a positive covariation in spatio-temporal densities between Pacific Herring and several groundfish species (i.e., Arrowtooth Flounder, Sablefish, Pacific Halibut, Pacific Cod), which highlights the need for an accurate assessment of the relative contribution of those species to the total summer predation pressure experienced by Pacific Herring off the WCVI. The findings of this study contribute to a better understanding of the WCVI marine ecosystem.