A fishery-independent survey of juvenile shortfin mako and blue sharks in the Southern California Bight
Published on 23. June 2016
A fishery-independent survey of juvenile shortfin mako (Isurus oxyrinchus) and blue (Prionace glauca) sharks in the Southern California Bight, 1994–2013
Rosa Runcie, David Holts, James Wraith, Yi Xu, Darlene Ramon, Rand Rasmussen, Suzanne Kohin
A fishery-independent abundance survey was initiated by the National Marine Fisheries Service (NMFS) and the California Department of Fish and Game (CDFG) in 1994 to track the relative abundance and size of juvenile shortfin mako (Isurus oxyrinchus) and blue (Prionace glauca) sharks in the Southern California Bight (SCB). The survey was designed based on data from an experimental commercial shark longline fishery that operated in the SCB during 1988–1991. Survey sets were conducted annually during the summer months within seven 10 × 10 min spatial blocks in the SCB close to the California Channel Islands. Between 1994 and 2013, survey effort totaled 460 sets. The standardized catch-per-unit-effort (CPUE) of shortfin mako showed a generally declining trend from 1994 through 2010 with increases during the final years to a level similar to those of the mid-1990s. For blue sharks, the standardized CPUE showed a generally declining trend throughout the time series with the lowest values in 2012 and 2013, and an anomalously high CPUE in 2000. Catch rates varied across the survey area with fewer, larger sharks caught in the more northern blocks of the survey. Sharks of age classes 0–2 represented the majority of those caught in the survey (approximately 81% of blue sharks and 58% of makos), yet there was variability in the sizes of blue and mako sharks caught by year. The sex ratio of age-0 sharks caught was not different from 1:1 for shortfin makos, but skewed toward females for blue sharks. Although the survey area is relatively small and results show interannual and spatial variability in CPUE that is not fully understood, these data represent the first and only fishery-independent survey that has targeted these shark species in the SCB. The results of the survey and associated data provide useful information regarding regional relative abundance, size- and sex-compositions, and spatial distributions of shortfin mako and blue sharks, all of which are essential for stock assessments and fisheries management.
Fisheries Research, Volume 183, November 2016, doi:10.1016/j.fishres.2016.06.010