Relationship between abundance of juvenile rockfishes and environmental variables documented off northern California and potential mechanisms for the covariation

Fishery Bulletin, Jan, 2007 by Thomas E. Laidig, James R. Chess, Daniel F. Howard

Abstract--We estimated annual abundance of juvenile blue (Sebastes mystinus), yellowtail (S. flavidus), and black (S. melanops) rockfish off northern California over 21 years and evaluated the relationship of abundance to oceanographic variables (sea level anomaly, nearshore temperature, and offshore Ekman transport). Although mean annual abundance was highly variable (0.01-181 fish/ minute), trends were similar for the three species. Sea level anomaly and nearshore temperature had the strongest relationship with interannual variation in rockfish abundance, and offshore Ekman transport did not correlate with abundance. Oceanographic events occurring in February and March (i.e., during the larval stage) had the strongest relationship with juvenile abundance, which indicates that year-class strength is determined during the larval stage. Also of note, the annual abundance of juvenile yellowtail rockfish was positively correlated with year-class strength of adult yellowtail rockfish; this finding would indicate the importance of studying juvenile abundance surveys for management purposes.

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Success of annual recruitment of juvenile rockfishes (Sebastes spp.) from the pelagic to a demersal phase is highly variable and leads to large fluctuations in year-class strength in the fisheries (Ralston and Howard, 1995). With the abundance of many rockfish stocks at historic low levels, assessment of recruitment is paramount for fishery managers to improve plans for sustainable harvests. Fisheries models (such as the stock synthesis model) often include recruitment data (PFMC (1)). In recent years, larval and juvenile rockfish abundance data have been included in stock assessments for bocaccio (S. paucispinis) and cowcod (S. levis) (MacCall (2), Butler et al., 2003). If the larval or juvenile abundance data do not accurately reflect the numbers of adults, the models can either over- or underestimate the biomass available to the fishery.

Rockfishes are viviparous, giving birth to larvae that reside in open water for several months before recruiting from the plankton to a more benthic life (Love et al., 2002). The habitat where rockfish initially recruit varies among species and includes floating drift algae (e.g., split-nose rockfish, S. diploproa [Shaffer et al., 1995]), soft sediments (e.g., stripe-tail rockfish, S. saxicola [Johnson et al., 2001]), and deeper rocky areas with crevices (e.g., rosy rockfish, S. rosaceus [Love et al., 2002]). A major area of recruitment is the nearshore environment, where numerous species of rockfish recruit in the kelp canopy, sand channels, on rocky outcrops, and in midwater among the kelp stipes (Anderson, 1983).

The factors that influence the magnitude of juvenile recruitment of rockfishes are not well known. VenTresca et al. (3) suggested that upwelling was the major factor contributing to the annual variability of juvenile rockfish abundance off Monterey, California. Ralston and Howard (1995) found that the highest survival of blue (S. mystinus), and yellowtail rockfish (S. flavidus), occurred when sea surface temperature and upwelling were at intermediate levels. Ainley et al. (1993), using seabird diet to assess the abundance of juvenile rockfishes, also reported the greatest abundance of rockfishes in years of intermediate upwelling. Yoklavich et al. (1996) and Johnson et al. (2001) observed that, during years when upwelling was delayed, those rockfishes released later in the year had increased survivorship. In addition, Pasten et al. (2003) observed that monthly tidal cycles were important in estimating the numbers of newly recruited black rockfish (S. inermis) in the western Pacific.

In our study, we examined the relationship between the annual abundance of juvenile rockfishes recruiting to the nearshore kelp beds off northern California and three oceanographic variables (sea level anomaly, nearshore temperature, and offshore Ekman transport) over 21 years. Three species of juvenile rockfishes (blue, yellowtail, and black rockfish, [S. melanops]) were surveyed. Parturition (the release of larvae from the mother) in these species occurs in the winter and the resulting pelagic larvae and juveniles spend between three to five months in the offshore midwater environment (Love et al., 2002). All juveniles of these species migrate in mid to late spring and early summer from the offshore pelagic environment to nearshore midwater and benthic environments on or next to rocky outcrops, commonly with dense algal growth; this movement we define here as juvenile recruitment (Anderson, 1983).

Materials and methods

Surveys of young-of-the-year (YOY) rockfishes were conducted annually between 1 July and 15 September from 1983 to 2003 within two kelp beds along the northern California coast (Dark Gulch [39 [degrees]14'N; 123 [degrees]46'W] and Salmon Point [39 [degrees]12'N; 123 [degrees]46'W]) in Mendocino County (Fig. 1). Each kelp bed included high-relief bedrock interspersed with low-relief cobble and sand areas. They were bounded by shoreline on two sides, a sand channel on one side, and deep water on the remaining side, and thus each kelp bed was isolated.