Resource Assessment & Conservation Engineering (RACE) Division
Newport Laboratory: Fisheries Behavioral Ecology Program
Fisheries Behavioral Ecology Program Initiates Research with Pacific Cod
Figure 1. Pacific cod eggs (~1 mm diameter) at 4°C – 5 days post-fertilization. Photo by Benjamin Laurel.
Figure 2. Mortality rates of unfed early-, mid- and late-hatched (days post hatch (DPH)) Pacific cod larvae as a function of temperature (0°-8°C).
Figure 3. Pacific cod larva (4.2 mm TL) at 8°C – 2 days post-hatch. Photo by Benjamin Laurel.
Figure 4. Pelagic juvenile Pacific cod (12 mm TL) at 8°C – 53 days post-hatch. Photo by Benjamin Laurel.
Figure 5. A juvenile cod collected in a Laminaria bed around Kodiak Island during July. Photo by Benjamin Laurel.
Pacific cod represent a commercially and ecologically important groundfish in Alaska waters. From the late 1970s to the early 1980s, Pacific cod populations experienced rapid growth, and currently rank second in amount and value landed in the groundfish fishery. Extreme year classes in the Gulf of Alaska have been linked with environmental events such as El Niño Southern Oscillation, but the specific mechanisms of recruitment variation are poorly understood for this species.
In 2006, the Fisheries Behavioral Ecology Program (FBEP) began a multiyear research program to examine the growth, survival, behavior and habitat associations of Pacific cod during their early life history.
Egg stages: In April 2006, Alisa Abookire (AFSC Kodiak Laboratory) coordinated the collection, fertilization and shipment of Pacific cod eggs (Fig. 1) from Chiniak Bay, Alaska, to FBEP’s laboratory in Newport, Oregon. Eggs were divided and acclimated among replicate tanks to provide five temperature treatments (0°, 2°, 4°, 6°, 8° C). Using this experimental framework, eggs were allowed to develop so that we could address two components: 1) temperature-mediated development rates of Pacific cod eggs in the Gulf of Alaska and 2) mechanisms that drive such patterns.
The first component has direct application for both estimating daily egg production in Alaska waters and for developing individual based models (IBMs) of early life history stages. Dry mass, morphometric measures, egg staging, and mortality rates were derived on a degree-day based sampling schedule. Hatch duration was also measured by counting and removing newly hatched larvae from egg incubators every 2-3 days. Results showed that peak hatch dates can range from 18 to 50 days post fertilization (DPF) at 8°C and 0°C, respectively. Pacific cod eggs survived and hatched at all the temperatures tested, but hatching success was significantly lower at 0°C relative to temperatures greater than or equal to 2°C.
How temperature affects egg development is being examined by lipid/fatty acid analysis. The most important source of stored energy in fish eggs are lipids, generally in the form of triacylglycerols (TAGs). However, cod species, such as Atlantic cod, are often TAG-deficient, thereby putting a dual demand on phospholipids (PL) for both energy and structure during embryonal development. We suspect temperature will be an important factor in how efficiently PL is catabolized and subsequently available for construction of essential cellular membranes. Lipid extractions and data analysis are being assisted by graduate student Louise Copeman.
Larvae: The effects of temperature were further examined during the prefeeding larval stage (Fig. 2) and (Fig. 3). In this experiment, we measured the consequences of early and late hatching at varying temperatures using larvae hatched from the previous egg experiments. Larvae hatched from these experiments were separated into early (0-3 DPF), mid (4-6 DPF) and late (7-9 DPF) hatching to follow their growth and survival trajectories to 50% mortality. This point in cumulative mortality is often referred to as the “Point of No Return” (PNR), the period at which remaining larvae are too weak to feed even if prey were available.
Temperature-mediated PNR relationships are shown in Figure 2 for early and late hatching larvae. In all temperature treatments, the early hatching fish were smaller but had more lipid reserves and survived longer in the absence of food than late hatching fish.
Pelagic juveniles: Some of the larvae hatched from the initial egg experiments were reared to pelagic juveniles (~12-18 mm total length (TL)) (Fig. 4) to better understand the larviculture protocols for future experiments as well as initiate temperature-mediated growth trials for juveniles. These initial efforts proved to be successful—Pacific cod larvae grew well on enriched rotifers and were able to be weaned onto microparticulate food without using live Artemia as a transitional diet (Artemia are attractive to marine larvae but low in nutritional quality). FBEP scientist Tom Hurst is using these weaned juvenile cod to examine temperature-mediated growth rates during this developmental stage.
Demersal juveniles: Several members of FBEP traveled to Kodiak Island to initiate field work on age-0 juvenile cod (Fig. 5). In July, Ben Laurel, Al Stoner, Mara Spencer and Paul Iseri used seines, small pots, and baited cameras to perfect survey and handling techniques, as well as examine habitat associations. Of the three gear-types, seines and baited cameras proved to be the best means of surveying cod, especially when used in tandem; seines provided physical samples for collections, whereas the baited cameras sampled areas where the seine was unable to operate (e.g., deep water and rocky areas).
In late August, Tom Hurst joined the field team during a second trip to broaden the spatial and temporal scope of the survey. Preliminary analysis of field data indicate that shallow, nearshore macrophytes (i.e., Laminaria and eelgrass in <5 m of water) support high numbers of juvenile cod, but that aggregations of juvenile cod are also found in open habitats at greater depths (i.e., 10-60 m, particularly as the juveniles grow to less vulnerable sizes). The broad size range of fish during August (41-131 mm TL) and regional size variation are research topics that FBEP will expand in the future, possibly assisted by daily increment analysis of otoliths.
Juvenile cod collected in Kodiak were successfully transported to the Newport laboratory, where they are now being used in a series of habitat and growth experiments. Al Stoner is also examining the behavior of juvenile Pacific cod around baited gear to complement the baited camera work in the field.
By Benjamin Laurel
Modeling Vital Rates of Pacific Cod Larvae and Juveniles With Climate Change
With a grant from the North Pacific Research Board (NPRB), laboratory data will be used to develop spatially-explicit models of growth potential for Pacific cod larvae and juveniles in the Bering Sea with respect to climate change. Models will incorporate field data (available and modeled) on larval and juvenile distributions, temperature and primary productivity (SeaWiFS/MODIS Aqua data). The overall model structure is being constructed in such a way that it can be refined and improved as additional experimental and field data are gathered. NPRB Co-PIs Lorenzo Ciannelli and Mike Behrenfeld of Oregon State University are working closely with FBEP members in leading these modeling efforts.
By Benjamin Laurel