Sanctuary Integrated Monitoring Network
Monitoring Project

Midwater Trawl Pre-recruit Survey

Principal Investigator(s)

  • John Field
    NOAA Fisheries
Start Date: June 08, 1983

Monitoring the abundance of early life history stages of fishes and invertebrates can provide important indicators of impending recruitment, or year-class strength, to support population assessments and fisheries management. The National Oceanic and Atmospheric Administration (NOAA) National Marine Fisheries Service (NMFS) Southwest Fisheries Science Center (SWFSC) Fisheries Ecology Division's Groundfish Analysis Team has been conducting annual surveys off California in order to provide an index of recruitment strength for pelagic juvenile rockfish (Sebastes spp.), as well as support process and ecosystem studies. Other important species collected by the survey include juvenile and adult Pacific hake (Merluccius productus), juvenile lingcod (Ophiodon elongatus), northern anchovy (Engraulis mordax), Pacific sardine (Sardinops sagax), market squid (Loligo opalescens), and krill (Euphausiacea) as well as mesopelagic species such as myctophids (Myctophidae). The most comprehensive list of organisms encountered throughout the temporal and spatial extent of the survey is provided in Sakuma et al. (2016), while Ralston et al. (2015) describe shifts in the forage assemblage relative to large scale climate factors as indicated by catches of some of the most frequently occurring species. In addition to abundance data, size information is also collected on species of interest. Genetic tissue samples and otoliths (used for daily aging) are also collected from the pelagic juvenile rockfish (e.g., Woodbury and Ralston 1991).

The survey began in 1983 sampling along the central California coast from Monterey Bay to Point Reyes. In 2004 the survey area was expanded from San Diego to Point Delgada and in 2013 it was further expanded to encompass the entire coast of California. Sampling period is in late spring (May-June), a time when the juveniles of many species of rockfish are still pelagic and have not yet settled to nearshore and benthic habitats. The first 16 years of the survey were conducted aboard the NOAA Ship David Starr Jordan. However, in 2008 this vessel was retired and from 2009-2015 a combination of NOAA ships and contract vessels was used for the survey. In 2016, the NOAA Ship Reuben Lasker (SWFSC replacement for the NOAA Ship David Starr Jordan) was used and will be the research platform going forward. A modified Cobb midwater trawl with a 26 m headrope and 9.5 mm codend mesh is towed at night (2100 to 0600 hr) for 15 minutes at a target headrope depth of 30 m.

In 1987 conductivity, temperature, and depth (CTD) casts were added to the survey to obtain oceanographic data. Over the years additional sensors have been added to the CTD including a fluorometer to measure chlorophyll, a PAR sensor and transmissometer to measure light levels and turbidity, and a dissolved oxygen sensor. Oceanographic data have been used in numerous studies, to characterize distribution and abundance of larvae and juvenile fishes, as well as many other organisms, relative to dominant physical oceanographic features (e.g., Sakuma and Ralston 1995, Schwing et al. 2000, Santora et al. 2012, Sakuma et al. 2013).

Additional data sources from the survey include marine mammal and seabird counts (starting in 1985), multi-beam acoustic sensor data used to map the distribution and abundance of krill (starting in 2000), and rod and reel collection of Humboldt squid (Dosidicus gigas) (starting in 2007). Marine mammal and seabird counts are recorded by biologists from Point Blue Conservation Science and the Farallon Institute for Advanced Ecosystem Research since the earliest years of the survey (data have been used in analyses by Ford et al. 2006, Santora et al. 2011a, Santora et al. 2012). Integration of multi-beam acoustic data with the krill catch data from the midwater trawls and the CTD data yields an informative overview of one of the key species that drives the ecosystem off the west coast (Santora et al. 2011b). Data collected on Humboldt squid include feeding habits, age and growth, and population genetics (Field et al. 2013, Hoving et al. 2013).

The variety of data collected aboard the survey (e.g., catch data, acoustic data, and hydrographic data) can be used not only to examine recruitment strength of various economically and ecologically important species, but also to study the general state of the ecosystem and its variability from year to year. Catch and CTD data are available for download on the ERDDAP website:

Summary to Date

YOY rockfish catches showed a large degree of temporal variability (Ralston et al. 2013). In addition, there is strong covariability in the temporal abundance patterns amongst many of the different species of rockfish. Catches of YOY rockfish were generally high 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. 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 relatively large numbers collected during 2002 and 2004. However, in 2005, catches north of Point Conception were very low. In addition, catches of other groundfish such as YOY Pacific hake were also at their lowest level in 2005 and were still quite low in 2006 north of Point Conception. However, in 2005 in the area south of Point Conception, catches of YOY rockfish and Pacific hake were extremely high indicating significant environmental/ecological differences between the regions north and south of Point Conception.

Catches of YOY rockfish and other groundfish were extremely high in 2013 coincident with generally cool ocean conditions. However, from 2014-2016, YOY rockfish and other YOY groundfish catches continued to by high even though there was a warming trend in ocean conditions. These warmer water conditions yielded high catches of more southerly distributed species, such as pelagic red crab (Pleuroncodes planipes), which are usually associated with El Nino events. Extremely high catches of pelagic tunicates such as salps (Salpidae) and pyrosomes (Pyrosoma atlanticum) also occurred during this time period. The co-occurrence of these species groups (i.e. temperate species like YOY rockfish, southern warm water species like pelagic red crabs, and the pelagic tunicates) was most pronounced in 2015, where all three species groups were extremely abundant (Sakuma et al. 2016).

Monitoring Trends

  • Catches of YOY rockfish were generally high 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.
  • YOY rockfish and other groundfish (e.g. Pacific hake) were extremely abundant from 2013-2016 with cooler ocean conditions in 2013 and warmer water conditions in 2014-2016.
  • The warmer water conditions observed in 2014-2016 also yielded large catches of species typically associated with El Nino conditions (e.g. pelagic red crabs), as well as large catches of pelagic tunicates. The co-occurrence of what could be considered three distinct species groups (i.e. temperate species like YOY rockfish, southern warm water species like pelagic red crabs, and the pelagic tunicates) was most pronounced in 2015, where all three species groups were extremely abundant.



Field, J.C., C. Elliger, K. Baltz, G. Gillespie, W.F. Gilly, I. Ruiz‐Cooley, D. Pearse, J.S. Stewart, W. Matsubu and W. Walker. 2013. Foraging ecology and movement patterns of the Humboldt squid (Dosidicus gigas) in the California Current. Deep Sea Research II 95: 37–51.

Ford, R.G., Ainley, D.G., Casey, J.L., Keiper, C.A., Spear, L.B. and Ballance, L.T., 2004. The biogeographic patterns of seabirds in the central portion of the California Current. Marine Ornithology, 32, pp.77-96.

Hoving, H. J. T., Gilly, W. F., Markaida, U., Benoit‐Bird, K. J., Brown, Z. W., P. Daniel, J.C. Field, L. Parassenti, B. Liu, and B. Campos. 2013. Extreme plasticity in life‐history strategy allows a migratory predator (jumbo squid) to cope with a changing climate. Global change biology 19: 2089–2103.

Ralston, S., K.M. Sakuma, and J.C. Field. 2013. Interannual variation in pelagic juvenile rockfish (Sebastes spp.) abundance -- going with the flow. Fisheries Oceanography 22(4):288-308.

Ralston, S., J.C. Field, and K.M. Sakuma. 2015. Long-term variation in a central California pelagic forage assemblage. Journal of Marine Systems 146:26-37.

Sakuma, K.M. and Ralston, S. 1995. Distributional patterns of late larval groundfish off central California in relation to hydrographic features during 1992 and 1993. CalCOFI Reports 36: 179-192.

Sakuma, K.M., Bjorkstedt, E.P. and Ralston, S., 2013. Distribution of pelagic juvenile rockfish (Sebastes spp.) in relation to temperature and fronts off central California. CalCOFI Reports 54: 167-179.

Sakuma, K.M., J.C. Field, N.J. Mantua, S. Ralston, B.B. Marinovic, and C.N. Carrion. 2016. Anomalous epipelagic micronekton assemblage patterns in the neritic waters of the California Current in spring 2015 during a period of extreme ocean conditions. CalCOFI Reports 57:163-183.

Santora, J.A., Ralston, S. and W.J. Sydeman. 2011a. Spatial organization of krill and seabirds in the central California Current. ICES Journal of Marine Science 68:1391–1402.

Santora, J.A., W.J. Sydeman, I.D. Schroeder, B. Wells and J. Field. 2011b. Mesoscale structure and oceanographic determinants of krill “hot spots” in the California Current: implications for trophic transfer and conservation. Progress in Oceanography 91: 397-409.

Santora, J.A., J.C. Field, I.D. Schroeder, K.M. Sakuma, B.K. Wells and W.J. Sydeman. 2012. Spatial ecology of krill, micronekton and top predators in the central California Current: implications for defining ecologically important areas. Progress in Oceanography 106: 154-174.

Schwing, F.B., Moore, C.S., Ralston, S. and K.M. Sakuma. 2000. Record coastal upwelling in the California Current in 1999. CalCOFI Reports 41: 148-160.

Woodbury, D. and S. Ralston. 1991. Interannual variation in growth rates and back-calculated birthdate distributions of pelagic juvenile rockfishes (Sebastes spp.) off the central California coast. Fishery Bulletin 89: 523-533.

Study Parameters

  • Age & Growth
  • Dispersal & Recruitment
  • Mortality
  • Morphology
  • Diversity
  • Disturbance
  • Biomass
  • Abundance
  • Distribution
  • Density
  • Age structure
  • Size structure
  • Stock assessment
  • Temperature
  • Density
  • Salinity
  • Upwelling/downwelling
  • Conductivity
  • Chl A
  • Currents
  • Wind

Study Methods

Annual 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) referred to as the core region, 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 to provide greater geographic coverage, expanding southward to San Diego (32o43’ N) and northward to Delgada (39o50’ N), just south of Cape Mendocino. The geographic coverage was again expanded in 2013 with stations sampled north of Cape Mendocino up to Flint Rock Head (41o30’ N). The current survey design can be categorized into five regions: north (north of Cape Mendocino), north central (between Cape Mendocino and the core region), core (historical sampling region), south central (between the core region and Point Conception), and south (south of Point Conception).

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. Acoustic data (EK500 initially and currently using an EK60) were collected continuously on each cruise starting in 2000. 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.

Beginning in 1985, marine mammal and seabird counts were recorded by biologists from Point Blue Conservation Science and the Farallon Institute for Advanced Ecosystem Research.

Figures and Images

Figure 5. 2016 map displays SWFSC’s sampling stations or survey transect points located within Monterey Bay NMS.


  • Data table for 2016
    Table 1. Total Count of individuals (Total Count) by species for 2016 SWFSC trawl surveys in Monterey Bay NMS.