Midwater Trawl Pre-recruit Survey
- Steve Ralston
Rockfish (genus Sebastes) exhibit extreme variability in reproductive success. For example, interannual variation in recruitment to the exploitable stock of bocaccio (Sebastes paucispinis) ranges over two orders of magnitude. As a consequence of that variability, the productivity of rockfish fisheries depends almost exclusively on the occurrence and influx of strong year-classes. Sound management of these fisheries, therefore, requires accurate information on impending recruitment. To meet that need, the NOAA NMFS SWFSC Fisheries Ecology Division has performed an annual survey of the distribution and abundance of pelagic juvenile young-of-the-year (YOY) rockfishes prior to their recruitment to the adult phase of the population. The surveys also collect similar information on the YOY of other commercially important species such as Pacific whiting (Merluccius productus), as well as data on the distribution of other prominent species within the California Current including northern anchovies (Engraulis mordax), Pacific sardines (Sardinops sagax), market squid (Loligo opalescens), and krill (Euphausiids).
Surveys have been fielded aboard the NOAA R/V David Starr Jordan during May/June every year since 1983. The survey samples a series of fixed trawl stations located off the California coast using a midwater trawl. The midwater trawl survey gear captures significant numbers of approximately 10 rockfish species during their pelagic juvenile stage (i.e., 50-150 days old), by which time annual reproductive success has been established. Catch-per-unit-effort data from the survey are analyzed and serve as the basis for predicting future recruitment to rockfish fisheries. Results for several species (e.g., bocaccio, chilipepper [S. goodei], and widow rockfish [S. entomelas]) have shown that the survey data can be useful in predicting year-class strength in age-based stock assessments.
The survey’s data on YOY Pacific whiting has also been used in the stock assessment process. To assist in obtaining additional northward spatial coverage of YOY Pacific whiting off Oregon and Washington, in 2001 the Pacific Whiting Conservation Cooperative in cooperation with the NOAA NMFS Northwest Fisheries Science Center began a midwater trawl survey patterned after the NOAA NMFS SWFSC Fisheries Ecology Division’s existing survey. Both surveys work cooperatively together each year in order to resolve interannual abundance patterns of YOY rockfish and Pacific whiting on a coastwide basis, which provides expedient, critical information that can be used in the fisheries management process.
Another collaboration has been with the Point Reyes Bird Observatory, which deploys ornithologists aboard to census seabirds and marine mammals during the annual surveys. Data gathered by these biologists have been widely used in understanding the distribution and foraging behaviors of seabirds off California.
The large quantity of physical data collected during the surveys (e.g., CTD with attached transimissometer and fluorometer, thermosalinometer, and ADCP) have provided a better understanding of the hydrographic conditions off the California coast and analysis of these data have been distributed through the publication of NOAA NMFS Technical Memoranda.
Summary to DateYOY rockfish catches showed a large degree of temporal variability. In addition, the temporal patterns were similar amongst many of the different species of rockfish. Catches were generally large during the 1980's, but dropped substantially during the 1990's with the exception of 1991 and 1993. The poor catches during the 1990's could be attributed to the generally warm conditions during that time period, and the occurrence of strong El Nino and La Nina conditions which adversely affect rockfish recruitment. Some of the lowest catches of YOY rockfish occurred in El Nino years, most likely because of the suppression of coastal upwelling and the associated drop in primary productivity.
Generally cooler ocean conditions began to develop beginning in 2000 and YOY rockfish catches increased substantially from the very low catches of 1998 (an El Nino year) with large numbers collected during 2002 and 2004. However, since 2004, catches north of Point Conception have been very low. In addition, catches of YOY Pacific whiting were also at their lowest level in 2005 and were still quite low in 2006 north of Point Conception. In 2005 in the area south of Point Conception, catches of YOY rockfish and Pacific whiting were extremely high indicating significant environmental/ecological differences between the regions north and south of Point Conception.
- Catches of YOY rockfish were generally large during the 1980's, but dropped substantially during the 1990's with the exception of 1991 and 1993.
- The lowest catches of YOY rockfish generally occurred in El Nino years most likely because of the suppression of coastal upwelling and the associated drop in primary productivity.
- YOY rockfish catches increased substantially from the low catches of 1998 with large numbers collected during 2002 and 2004. However, catches within the same geographic region dropped dramatically in 2005 and 2006.
- While catches of YOY rockfish and Pacific whiting dropped dramatically in 2005 in the region north of Point Conception, extremely large catches occurred south of Point Conception showing that there are significant spatial differences in YOY distribution.
- Age & Growth
- Dispersal & Recruitment
- Age structure
- Size structure
- Stock assessment
- Chl A
Study MethodsAnnual cruises aboard the NOAA R/V David Star Jordan began in 1983 and have been conducted during May/June off California, a time when most YOY rockfish are identifiable to species and have not yet settled to nearshore and benthic habitats. Throughout this time, a standard trawl consisted of a 15-minute nighttime tow of a large midwater trawl (26 m headrope and 9.5 mm codend mesh) set to a target headrope depth of 30 m. Additional tows were made at other depths (i.e., 10 and 100 m) as allowed by constraints imposed by time and bottom bathymetry. In 1986, the sampling design was altered to permit three consecutive "sweeps" through an area bounded by Cypress Point off southern Monterey Bay (36o35’N) and Point Reyes (38o10’N), and from the coast to about 75 km offshore. Five or six stations along a transect were sampled each night and seven transects were completed for each sweep. Beginning in 2004 the sampling design was altered again to provide greater geographic coverage, expanding southward to San Diego (32o43’N) and northward to Delgada (39o50’ N), just south of Cape Mendocino. Beginning in 1987, a CTD cast was conducted at each nighttime trawl station occupied. In addition, daytime activities were restructured to permit sampling of a new grid of standard CTD stations. Standard CTD stations were specific locations where CTD casts were scheduled and repeated for each cruise. The CTD rate of descent was 45 m/minute to a depth 10 m off the bottom down to a maximum depth of 500 m. In 1994 a photosynthetically available radiation (PAR) sensor was added to the CTD, in 1996 a fluorometer was added, and in 1997 a transmissometer was added. Beginning in 1998 a carousel and conducting cable were used with the CTD to allow for real-time data acquisition. Additional sources of hydrographic data included the vessel's thermosalinometer (TS) unit that provided a continuous data stream of surface temperature and salinity, and a bench-top fluorometer used to measure chlorophyll a levels at the surface. Collection of TS data started in 1988 and bench-top flurometer data collection started in 2002. Beginning in 1993 an acoustic Doppler current profiler (ADCP) was operated continuously on each cruise and in 1999 EK500 data collection was added. In addition to the standard nighttime trawls and daytime/nightime CTDs, when time allowed, discretionary sampling was conducted to focus on specific bathymetric features, such as Cordell Bank or Pioneer Canyon, or devoted to the intense study of oceanic features or processes that may be key to successful recruitment.
Figures and Images
Midwater trawl pre-recruit survey stations.
Annual midwater trawl surveys have been conducted aboard the National Oceanographic Atmospheric Administration (NOAA) research vessel David Starr Jordan.
Deck crew of the NOAA R/V David Starr Jordan working with the midwater trawl net.
Scientists sorting the catch from the midwater trawls in the aft laboratory of the NOAA R/V David Starr Jordan.