Pacific capelin Mallotus catervarius are planktivorous small pelagic fish that serve an intermediate trophic role in marine food webs. Due to the lack of a directed fishery or monitoring of capelin in the Northeast Pacific, limited information is available on their distribution and abundance, and how spatio-temporal fluctuations in capelin density affect their availability as prey. To provide information on life history, spatial patterns, and population dynamics of capelin in the Gulf of Alaska (GOA), we modeled distributions of spawning habitat and larval dispersal, and synthesized spatially indexed data from multiple independent sources from 1996 to 2016. Potential capelin spawning areas were broadly distributed across the GOA. Models of larval drift show the GOA’s advective circulation patterns disperse capelin larvae over the continental shelf and upper slope, indicating potential connections between spawning areas and observed offshore distributions that are influenced by the location and timing of spawning. Spatial overlap in composite distributions of larval and age-1+ fish was used to identify core areas where capelin consistently occur and concentrate. Capelin primarily occupy shelf waters near the Kodiak Archipelago, and are patchily distributed across the GOA shelf and inshore waters. Interannual variations in abundance along with spatio-temporal differences in density indicate that the availability of capelin to predators and monitoring surveys is highly variable in the GOA. We demonstrate that the limitations of individual data series can be compensated for by integrating multiple data sources to monitor fluctuations in distributions and abundance trends of an ecologically important species across a large marine ecosystem.
Nocturnal distributions and habitat preferences of juvenile fish along urban shorelines are understudied relative to daytime investigations. As a case study, nocturnal distributions of juvenile Pacific salmon (Oncorhynchus spp.) among ecological engineered and conventional seawall and pier habitats were characterized from May through August 2019 along the Seattle, WA, USA, waterfront. A multibeam sonar mounted beneath a kayak enabled day-night fish density comparisons to identify distribution and habitat use differences. Ecological engineering included mattresses (mesh bags filled with rocks) to create intertidal benches, a textured seawall to increase invertebrate colonization, and embedded glass blocks in an overhanging sidewalk to increase ambient light. Overall juvenile salmon night presence was twice that of daytime, and counts were 1.5 times higher during peak salmon densities. Juvenile salmon presence in eco-engineered and reference habitats was more similar at night compared to day. At night, juvenile salmon avoided traditional under-pier habitats and were more likely to navigate around piers that lack nearshore eco-engineered habitats. Increased use of eco-engineered habitats by juvenile salmon at night highlights the need to incorporate diel fish distribution differences in resource abundance estimates, and in the design and construction of coastline modifications.
A mobile dual-frequency identification sonar (DIDSON) was used to characterize juvenile Pacific salmon (Oncorhynchus spp.) daytime use of armored and eco-engineered seawall habitats along an urbanized shoreline in Puget Sound, Washington, USA. Eco-engineering included intertidal benches to elevate the seafloor, a textured seawall to provide refuge and encourage invertebrate colonization, and glass blocks in an overhanging sidewalk to increase ambient light. A DIDSON multibeam sonar gave salmon counts twice that of visual surveys, and was thus deemed effective as a mobile sampling tool for small fish (~40-90 mm) and can be advantageous relative to visual methods depending on research goals, habitats, and ambient light levels. Increased salmon presence in the eco-engineered intertidal corridor relative to traditional seawall and pier habitats showed that the combination of increased light, reduced infrastructure (e.g. pier pilings), increased texture, and a shallower seafloor improves habitat function for juvenile salmon. High densities of juvenile salmon along pier ends show that salmon also use alternative migration pathways, with unknown energetic costs and predations risks.
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