Relationships between water mass characteristics and estimates of fish population abundance from trawl surveys

Abstract

The Canadian Department of Fisheries and Oceans conducts annual bottom trawl surveys to monitor changes in the abundance of the major commercially important groundfish populations. Some of these surveys have been in operation for almost 20 yr. The estimates from these surveys often indicate rapid changes in abundance over time beyond that expected from the population dynamics of the fish. Much of this interannual change has been interpreted as variation, the magnitude of which has often made it difficult to measure anything but the most severe effects of fishing, pollution or any other intervention on the population. Recent studies have shown that some of this variation may be attributed to changes in catchability of fish due to the effects of environmental variables on fish distribution. Annual changes in abundance as estimated from such field surveys may be confounded by changes in catchability due to annual changes in environmental conditions. In this study, trawl catches of age 4 Atlantic cod (Gadus morhua) from surveys conducted during March 1979-1988 were compared with concurrent measurements of bottom salinity, temperature and depth. Large catches of age 4 cod are more likely to occur in water characterized as the intermediate cold layer defined by salinities of 32-33.5 and temperatures<5°C. This relationship also appears to be modified by depth. We further show that internnual changes in the estimated abundance from the surveys were, in a number of cases, coincident with changes in the proportion of the bottom water composed of the intermediate cold water layer. The implications that these patterns may have on interpreting trends in the estimates of abundance from trawl surveys are discussed.

Density‐independent and density‐dependent factors affecting temporal changes in spatial distributions of eastern Bering Sea flatfish

“…This indicates some plasticity of EBS flatfish with respect to water temperature, and their habitat selection on summer feeding grounds may be more strongly influenced by prey availability and/or predator avoidance than maintaining their populations within a water mass with particular temperature characteristics. In contrast, other studies of habitat use for EBS groundfish found that the distributions of Arctic cod and walleye pollock respond strongly to temperature variability (Wyllie‐Echeverria and Wooster, 1998), and Atlantic cod on the Scotian shelf were associated with a water mass associated with a particular temperature and salinity range (Smith et al. , 1991).…”

“…Temperature variability can affect research trawl surveys either through availability (from changes in spatial distributions) or catchability (from changes in the behavioral response of fish to survey gear), thus posing significant management challenges. For example, Smith et al. (1991) found that the availability of Scotian shelf Atlantic cod to a bottom trawl survey was a function of the extent to which an intermediate layer water mass extended to benthic habitats, thus confounding interpretation of survey abundance estimates.…”

Density-Dependent Geographic Distribution of Atlantic Cod (Gadus morhua) in the Southern Gulf of St. Lawrence

“…The factors that determine habitat Smith et al 1991;Sinclair 1992;Swain 1993). However, it is quality for cod are not well known.…”

Environmental influences on the availability of smooth pink shrimp, Pandalus jordani, to commercial fishing gear off Vancouver Island, Canada

“…The combined effects of winds and tides from specific directions and water‐mass changes may affect the local spatial distributions of shrimp, such that they may be concentrated into fewer but denser aggregations under certain conditions but more widely dispersed under other conditions. This would increase the variability of catch rates and survey abundance estimates under the aggregated condition, as has been proposed for cod in the north‐west Atlantic ( Smith et al ., 1991 ). There is some initial support for this idea, as preliminary spectral analyses of the CPUE correlogram data (not shown here) indicate a significant peak at 16 h, which is the period for the inertial water movements (15.9 h) that would be induced in this area by surface winds.…”

“…Understanding variations in catch rates in response to variations in environmental conditions is therefore an important component of improving stock assessment models and fisheries management advice, especially for reducing the variance of abundance indices that are derived from research surveys (e.g. Smith et al ., 1991 ; Perry and Smith, 1994). However, surveys do not usually collect enough replicate tows at the same location to evaluate short‐term variations in availability; commercial fishery data must generally be used for this purpose.…”