Broad-Scale Report

BROAD-SCALE SURVEY PROJECT

[This report was prepared by P. Wiebe (WHOI) and D. Mountain (NMFS-Woods Hole]

STATEMENT OF OBJECTIVES

The primary objective of the US GLOBEC program is to understand the underlying physical and biological processes that control the population dynamics of the pelagic early life stages of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) and the copepod zooplankton, Calanus finmarchicus and Pseudocalanus spp. The gadid fishes are traditionally among the most commercially valuable stocks in the Georges Bank region, and Calanus and Psuedocalanus are both important prey organisms for the larval and juvenile gadids. The planktonic early life stages are important since population levels are believed determined, to a large extent, during this period. It is also the period when the organisms are most directly influenced by the local physical environment and thus most susceptible to the effects of a changing climate.

The GLOBEC NW Atlantic/Georges Bank field program, which began in 1994, has three basic components of the program: broad-scale field studies, fine-scale process studies, and modelling/historical data studies.

Within the broad-scale field studies, a group is dedicated to making basic descriptions of the primary physical and biological properties of the Bank and the surrounding waters, as well as their spatial and temporal variability. This group focuses specifically on the acquisition of this information through intensive shipboard surveys of the region. The primary focus is on documenting the life histories of the target species (cod, haddock, Calanus, and Pseudocalanus) in relation to the physical oceanographic conditions on the Bank. The sampling and processing conducted by this group are providing the samples and information for many associated analyses by collaborating investigators which are described in separate documents.

Two approaches to study the life history and population dynamics of the target species are being used. The first is a life table or cohort analysis. The abundances of successive stages in the life history of the population are determined and mortality rates between stages are calculated directly. These rates can be compared to likely mortality mechanisms (e.g., predation, starvation, advective loss/ exchange). To do this requires frequent, intensive sampling, along with measurements of growth rate and/or development time. The second approach is survivorship analysis. This requires less intensive sampling in which the characteristics of individuals are measured in several life stages to determine the characteristics which favored survival (e.g., birthdate, growth rate, genetic parentage). Both cohort and survivorship analyses can be used in this study because of: 1) the relatively discrete nature of the cod/haddock spawning on Georges Bank (in both space and time); 2) the relatively higher abundance of Calanus and Pseudocalanus on Georges Bank than in surrounding waters; 3) the general retention of plankton within the confines of the Bank; 4) the relatively small size of the Bank; and 5) the high probability of a cohort age structure for Calanus finmarchicus driven by a large, discrete egg production "event" associated with the spring bloom.

STATEMENT OF WORK

There have been six broad-scale cruises, one in June 1994 and five thus far in 1995. The latter is series of ~12-day cruises which have occurred at approximately monthly intervals beginning in February, 1995. The 1995 series will be completed by one more cruise aboard the R/V Albatross IV in July. These Broad-scale cruises have been interwoven with process-oriented cruises and cruises to deploy, tend, and retrieve moorings. The specific objectives of these cruises were:

To conduct a Broad-scale survey of the distribution and abundance of target species on Georges Bank, including: larvae and juveniles of cod and haddock; Calanus finmarchicus, Pseudocalanus spp., and their predators and prey;
To conduct a hydrographic survey of the Bank;
To collect Calanus finmarchicus and Meganyctiphanes norvegica for population genetic studies;
To launch drifter buoys for circulation studies on the Bank;
To map the Bank-side velocity field using an ADCP.
On some cruises, to conduct acoustic mapping of the plankton along the tracklines between stations using a high frequency echo sounder deployed in a towed body.

The Broad-scale Survey consists of 38 standard stations which have a fixed geographical location. The stations are of two types. "Full" stations involve plankton pump deployment, profile of the CTD- fluorometer with water bottle rosette, double oblique 1-m2 MOCNESS tow, single oblique 10-m2 MOCNESS tow, and bongo net tow. "Partial" stations omit the pump deployment and 10-m2 MOCNESS tow. There are 18 full stations and 20 partial stations on the survey plan. Following the completion of the standard stations and depending upon the findings from ship board analyses of the samples, additional stations were occupied to enhance the sampling resolution in key locations.

SUMMARY OF KEY FINDINGS:

A description of key findings for most aspects the broad-scale cruises can be found in the individual reports of the investigators. The MOCNESS samples for ichthyoplankton analyses are awaiting funds to support their sorting and counting. This sorting affects the determination of the distribution and abundance of gadid eggs and larvae, analysis of the gut contents of the gadid larvae, and analysis of the gadid otoliths for aging and mortality. The nutrient samples that have been collected are in freezer storage. Processing them cannot begin until the funds are forthcoming.