UC San Diego

Most organisms shift between different ecological niches or habitats throughout their lives. These shifts are prompted by growth and changing resource needs. In the marine realm, understanding why and when fish shift habitats is particularly important due to the increasing use of spatial management as a conservation strategy. Effective spatial management requires understanding how a species habitat usage changes throughout its’ lifespan.

Within the Southern California Bight (SCB)Yellowtail (Seriola lalandi) are iconic gamefish and widely targeted throughout the region, both in U.S. and Mexican territorial waters. Their cross-border movements mean these fish encounter a diverse array of anthropogenic pressures, ranging from ocean-warming to agricultural and urban run-off to significant recreational, artisanal, and commercial fishing.

This work attempts to understand and quantify how yellowtail use the Southern California Bight and how that usage affects their biology. This was carried out in 3 separate chapters dealing with analysis of long-term recreational catch records, conventional tagging, passive acoustic telemetry as well as spatially-explicit analysis of age, growth, diet and trophic position.

The primary differences detected across all investigated parameters were size-mediated. Thus, one contiguous population with distinct ontogenetic shifts in habitat and diet is the most parsimonious explanation for the results from each chapter presented in this thesis. Recreational catch data showed inshore and offshore catch sizes were different between years and across seasons and fish size, rather than tagging season best explained detection rates of acoustically tagged fish. These findings supported claims by recreational anglers that large fish caught inshore are potentially year-round SCB residents. The conclusion of one, panmictic, SCB yellowtail population is further supported by results from life-history analysis as fish size again was the only source of significant differences in age/growth, diet, or trophic position regardless of sampling location or region.

Results from tag returns, acoustic telemetry and life-history analysis indicate that there is likely one contiguous population of yellowtail in the SCB and that due to high

levels of fishing pressure, this population may be reliant on seasonal influxes of fish from the south to sustain current fishing levels.