Improving Estimates of Rockfish Populations

When the status of cowcod and other commercially important rockfishes (genus Sebastes) populations in southern California was estimated to be less than or equal to 25% by the Pacific Fisheries Management Council (PFMC) , great concern arose in regards to the potential impact of the recreational fishery on these populations. This concern led to the designation of a Cowcod Conservation Area (CCA; Figure 1) that closed off a large part of the Southern California Bight (SCB) to rockfish angling. The CCA was implemented in hopes to protect cowcod and their habitat so that the remaining fish have a chance to reproduce and rebuild the overall population.

Figure 1. Map of the collaborative acoustic and ROV survey sites. The green lines represent areas surveyed with acoustics. ROV surveys were also conducted in each of these area. The two Cowcod Conservation Areas (CCA) are indicated by the dashed line.

Information on rockfish populations within the SCB is limited due to the lack of studies aimed at tracking population densities and fish size distributions over time. Most of the information available is from intermittent fisheries catch data, which is not ideal for monitoring trends in abundance. For this reason it is crucial that rockfish populations be monitored within and outside of the CCA during the time of closure and thereafter. The challenge thus lies in the question of how to effectively and non-destructively monitor rockfish populations and assess habitat. Past surveys methods, which used gear like trawl nets, are no longer desirable due to the destructive nature of this gear type. Research at the SWFSC aims to quantify and obtain size estimates for rockfish and characterize rockfish habitat by means of sonar or acoustic surveys. This technology has been used for invertebrates and other fish species, but never successfully for rockfish.

Rosy rockfish (Sebastes rosaceus)

The development of acoustic survey methods is an ongoing process that has been improved upon over the last few years. The need to verify the results of acoustic surveys has been met by remotely operated vehicle (ROV) video surveys. Data on species compositions, abundance, and habitat associations is collected with the ROV. During 2004 and 2005 there was a concerted effort to survey a large part of the SCB during three acoustic cruises, followed by ROV cruises for verification of species composition and habitat.  Preliminary results show some interesting trends in fish populations, and promising accordance between acoustic survey results and ROV survey results.