No abstract

Quantifying the links between the marine environment, prey occurrence, and predator distribution is the first step towards identifying areas of biological importance for marine spatial planning. Events such as marine heatwaves result in an anomalous change in the physical environment, which can lead to shifts in the structure, biomass, and distribution of lower trophic levels. As central‐place foragers, seabirds are vulnerable to changes in their foraging grounds during the breeding season. We first quantified spatiotemporal variability in the occurrence and biomass of prey in response to an abrupt change in oceanography as a result of a marine heatwave event. Secondly, using multivariate techniques and machine learning, we investigated if differences in the foraging technique and prey of seabirds resulted in varying responses to changes in prey occurrence and the environment over a 2.5‐yr period. We found that the main variables correlated with seabird distribution were also important in structuring the occurrence and biomass of prey; sea‐surface temperature (SST), current speed, mixed‐layer depth, and bathymetry. Both zooplankton biomass and the occurrence of fish schools exhibited negative relationships with temperature, and temperature was subsequently an important variable in determining seabird distribution. We were able to establish correlations between the distribution of prey and the spatiotemporal distribution of albatross, little penguins and common‐diving petrels. We were unable to find a correlation between the distribution of prey and that of short‐tailed shearwaters and fairy prions. For high‐use coastal areas, the delineation of important foraging regions is essential to balance human use of an area with the needs of marine predators, particularly seabirds.

Euphausiids are a keystone species in coastal food webs due to their high lipid content and seasonally high biomass. Understanding the habitat and environmental drivers that lead to areas of high biomass, or ‘hotspots’, and their seasonal persistence, will support the identification of important foraging regions for mid- and upper- trophic level predators. We quantify the distribution of hotspots of the two dominant species of euphausiid in the north-east Pacific Ocean: Euphausia pacifica and Thysanoessa spinifera, as well as euphausiid larvae (mixed species). The Canadian coast encompasses the northern California Current Ecosystem and the transition zone to the Alaska current, and is a highly productive region for fisheries, marine mammals, and seabirds. We used spatiotemporal modelling to predict the distribution of these three euphausiid groups in relation to geomorphic and environmental variables during the important spring-summer months (April through September) when euphausiid biomass is highest. We quantified the area, intensity, and persistence of biomass hotspots across months according to specific oceanographic ecosections developed for marine spatial planning purposes. Persistent hotspots of both adult species were predicted to occur along the 200 m depth contour of the continental slope; however, differences were predicted on the shallower Dixon shelf, which was a key area for T. spinifera, and within the Juan de Fuca Eddy system where E. pacifica hotspots occurred. The continental slope along the west coast of Vancouver Island was the only persistent hotspot region common between both adult species and euphausiid larvae. Larval distribution was more correlated with T. spinifera than E. pacifica biomass. Hotspots of adults were more persistent across months than hotspots of euphausiid larvae, which were seasonally patchy. The persistence of biomass hotspots of forage species through periods of low overall biomass could maintain trophic connectivity through perturbation events and increase ecosystem resilience to climate change.

No abstract

The main objective of this study was to ascertain if the 2014 to 2016 spring marine heatwaves, had a significant effect on early summer fish assemblage species composition, abundance and biomass in eelgrass meadows. It was presumed that increased water temperatures above the 90th percentile of the 30y long-term spring temperatures would result in changes to fish growth, survival and ultimately lead to changes in early summer settlement to eelgrass. We assessed this by analyzing the catch per unit effort of 18 common fish species and total fish assemblage biomass in 22 eelgrass meadows from two study areas in the northern California Current during the early summer over an 18 year period from 2004 to 2021. A control chart analysis indicated that eelgrass fish assemblages during heatwave years (2015-16) were significantly higher than the 90th percentile in Bray Curtis dissimilarity of 13 baseline years. Furthermore, analysis of similarity indicated that about one half of the 18 fish species showed variable contributing increases or decreases in catch per unit effort during the heatwave period. In addition, the same fish families from the two study areas only 100km apart did not respond in synchrony to the MHW. We could not however detect any changes in eelgrass fish species composition or assemblage level biomass during the heatwave years. An analysis of the temporal trajectory over the 17y time series in the ordination plots also revealed that changes in fish assemblage dissimilarity during the 2015 and 2016 heatwave returned quickly to levels observed during baseline. Altogether, the results of this study suggest that while the short-term (34-42 d) spring heatwave events resulted in changes fish species abundances, these changes were short-lived. Hence, our results warrant caution about generalizing about the effects of heatwaves on nearshore marine fish.