NWA - Chapter 3

## GEORGES BANK FIELD STUDY: SUMMARY AND RECOMMENDATIONS

In this section we summarize the recommendations of the Canada/U.S. Meeting on N.W. Atlantic Fisheries and Climate, extracted from the detailed Working Group Reports, which are presented in their entirety in Section 9 of this document. These recommendations are intended to serve as guidelines for specific studies to be proposed under GLOBEC.

For each taxonomic group - fish, zooplankton, and benthos - specific recommendations are organized according to Site Selection criteria, with the exception that criteria relating to climate change are first discussed in general as applicable to all populations considered, and aspects of international, inter-program and inter-agency activity are discussed in a separate section following this one (Section 4). An outline of the general logistics of the field study is suggested in Section 6.

### 3.1 Relation to Climate Change

Georges Bank is an excellent location for the study of marine planktonic populations in the context of global climate change. Coupled ocean-atmosphere models suggest that the greatest anomalies in climatic conditions will occur in the North Atlantic Ocean. Georges Bank is situated near the confluence of two major ocean currents of significantly different origin - the subpolar Labrador Current with associated flows in the shelf and slope waters, and the tropical/subtropical Gulf Stream. The position and dynamics of these currents might be expected to be greatly altered by global climate change. Such a location is ideal for a GLOBEC field study.

It is assumed that changes in the recruitment of fish, holozooplankton and benthic species with planktonic larvae are rooted in the early stages of life. Thus we can only understand the links between recruitment and climatic change if the population parameters in those early stages are described and understood. The key questions identified with respect to climate change are:

1. Will global climate change produce the same events as in recent history, but in different places on the bank/shelf?

2. Will global climate change produce the same events as in recent history, but at different times within the annual cycle?

3. Will global climate change produce the same types of events, but of greater or lesser magnitude?

4. Will global climate change produce different types of events in the key populations?

5. How will global climate change alter physiology and behavior?

#### 3.1.1 Approach

The general approach is to undertake studies which will address the role that climate plays in determining local and regional episodic events, mass transport, and total energy of the marine system. A better understanding of ocean/atmosphere coupling, together with a better understanding of how physical mechanisms affect populations, will lead to the basis for predicting how climate change will affect population dynamics.

### 3.2 Fish

#### 3.2.1 Target species

The target fish species are recommended to include both cod and haddock. Cod is recommended because of the significant interest in cod populations throughout the North Atlantic. Haddock is worth considering as a related gadoid stock. Both cod and haddock populations on Georges Bank are depleted to the point that their future as the basis of viable fishery is in doubt. The interrelationships of these stocks to those on the Scotian Shelf and Grand Banks, where the haddock are also severely depleted, is an important issue requiring resolution.

#### 3.2.2 Definable populations

It is generally agreed that cod and haddock on Georges Bank are populations distinct from those in other regions of the North Atlantic (e.g., Scotian Shelf, Grand Banks, Barents Sea). The identification and study of the Georges Bank populations can only come from a study which is highly resolved in both space and time. The following specific studies are indicated:

1. Frequent surveys of Georges Bank and surrounding waters using sampling equipment which permits high spatial resolution in both the vertical and horizontal (e.g., MOCNESS, acoustics);

2. The use of molecular and biochemical techniques to characterize stock structure; and

3. Identification of key spawning sites and their characteristics.

#### 3.2.3 Population dynamics

A key objective would be to study changes in numbers of cod and haddock larvae along the 80-100 m isobath in late spring with respect to vertical stratification. A cohort of larvae could be followed from the spawning ground and sampled at frequent intervals. The following specific studies are suggested:

1. Growth rates of larvae in situ, using measurements such as changes in length, weight and daily growth rings of otoliths;

2. Comparative studies of growth rate relative to stratification and development of the seasonal thermocline;

3. Mortality estimates in the course of Lagrangian drifter experiments in which fish larvae are sampled at a high rate in space and time.

#### 3.2.4 Focus on process and mechanisms

Results of studies on processes will provide the principal insight into mechanisms controlling population dynamics. Specific processes recommended for study include:

1. Direct effect of temperature on growth rates, seasonal energy budgets and stage duration of life history stages;

2. Overwintering energetics;

3. Studies of food and feeding, with consideration of how this may change as a result of reasonable climate change scenarios;

4. Mechanisms by which adults locate spawning sites;

5. Basic temperature preferences and tolerances of the species, and the behavioral mechanisms used to select and maintain position in desirable water masses;

6. Mechanisms of retention and export on the banks;

7. The relation of early life history stages to substrate characteristics;

8. Effects of storms on behavior, distribution, and mortality.

#### 3.2.5 Historical database

There is a broad variety of historical databases available for both cod and haddock, together with ancillary data on environmental conditions. These databases should be explored, and other methods of extracting historical information should be developed. Specific studies might include:

1. Paleoecological studies on quantification of fish remnants (e.g., scales, otoliths) and associated paleoindicators such as 16O/18O ratios to provide long time series on populations;

2. Historical data on fish populations could be used to determine the degree to which Sequential Population Analyses smooth year-class variation; if year class variation is significantly smoothed, then individual population estimates may not be appropriate for determining relationships between populations and the environment;

3. Determination of the viability of hindcasting physics and biology from existing data sets on catches and temperature records which may be long, but of uneven and unknown quality;

4. Comprehensive analysis of data from groundfish surveys which have been conducted for decades on every bank from the New York Bight to Northern Labrador and Greenland. Particular attention should be paid to assessing how the pattern of aggregation varies with time, stock size and oceanographic conditions, and more use should be made of oceanographic data collected on these surveys;

5. Establishment of a Historical Data Working Group to locate data sets, to evaluate their quality, recommend the application of appropriate statistical tools, and make the data sets available to the community.

#### 3.2.6 Modeling

Specific modeling studies suggested in relation to populations of cod and haddock include:

1. Increased modeling of the physical oceanography of Georges Bank, focusing particularly on (i) dynamics of advection on the South Slope, and (ii) forcing functions with emphasis on storms;

2. Development of a full annual energy budget of each species, particularly cod;

3. Models which incorporate improvements with respect to assessing effects of commercial fishing, particularly the consequences of fishing pressure on fish biology;

4. Development of statistical methods which improve quantitative estimates of uncertainty associated with most measurements in fisheries and physical oceanography;

5. Development and/or application of statistical tools which better handle and interpret multidimensional patterns and nonlinear characteristics of biological and physical oceanographic data sets, because common regression techniques are inadequate;

6. Improvements in modeling of hydroacoustics and development of software to more rapidly process the enormous amount of data generated by acoustic instrumentation.

#### 3.2.7 Technology

Certain developments in technology recommended under the GLOBEC program will be applicable to all taxonomic categories of interest, particularly in the case of field sampling instruments. These general recommendations for new instrumentation are discussed in Section 5. Here we recommend the following advances in technology which would be of special value for studies of fish:

1. Improved biotechnological methods for assessing feeding rates and identifying food items;

2. Improved hardware and software for hydroacoustic techniques to locate, identify and quantify aggregations of fish, from larvae to adults;

3. Development of genetic markers useful in characterizing stock structure;

4. Development of methods which permit one to differentiate between environmental and heritable components of growth rate;

5. Fast, affordable tools for biogeochemical analysis of core samples;

6. Improved biochemical methods for measuring growth rate;

7. Innovative methods for measuring mortality.

### 3.3 Zooplankton

#### 3.3.1 Target species

Calanus finmarchicus is designated as the primary target species because (1) it dominates the zooplankton biomass on Georges Bank during the winter and spring; (2) it is an important item in the diets of larvae and pelagic juvenile stages of commercially important cod, haddock and herring; (3) abundant data are available on the physiology and ecology of Calanus spp.; and (4) reasonably good historical data sets are available for Calanus on Georges Bank.

Other species, notably Pseudocalanus spp. and Centropages spp., are also recommended for study, primarily because they dominate the copepod biomass in summer and fall, and also constitute an important fraction of the diet of larval fish on Georges Bank.

#### 3.3.2 Definable populations

It is not known whether the Calanus finmarchicus on Georges Bank form a distinct population, or whether they are in some sense continuous with the population(s) in the Gulf of Maine or on the Scotian Shelf. However, it is clear that C. finmarchicus must overwinter at depths which are greater than anywhere on the Bank, and it is supposed that the springtime flux of this species onto Georges Bank derives in large measure from animals which break out of their overwintering state and rise to the surface waters in the southwestern portion of the Gulf of Maine. In order to clearly define the C. finmarchicus population(s) on Georges Bank, the following study components are indicated:

1. Frequent surveys of the entire Bank, encompassing adjacent deep waters of the Gulf of Maine and Slope Waters, particularly in late winter and spring, to obtain high resolution samples in both the horizontal and vertical;

2. The use of biochemical and genetic marker techniques to clearly distinguish populations.

#### 3.3.3 Population dynamics

The primary objectives recommended for the holozooplankton component of the Georges Bank field study are to investigate the population dynamics of Calanus finmarchicus from January through June, in terms of birth, growth and death and in the context of physical transport processes on Georges Bank. Particular emphasis should be placed on understanding how the population is affected by the onset of seasonal stratification. The following specific studies are indicated:

1. Growth rate measurements which, as much as possible, use new techniques;

2. Comparative studies of growth and mortality relative to seasonal stratification, both above and below the thermocline;

3. Quantification of the effects of advection and diffusion on local rate of change in numbers;

4. In situ measurements of birth rate, particularly of the winter spawners.

Similar studies could be carried out on populations of Pseudocalanus spp. and Centropages spp.

#### 3.3.4 Focus on process and mechanisms

Process studies would be carried out primarily on cruises designed specifically for that purpose, in conjunction with investigations of processes affecting other taxa. Research would include the following studies:

1. Effect of food and feeding on in situ rates of growth, incorporating studies of moulting;

2. Processes which control overwintering;

3. Egg production rates estimated from both shipboard laboratory incubations as well as in situ, and the relation of reproductive activity to environmental variables;

4. Rates of mortality in relation to predation (which would require proper identification of predators);

5. Assessment of what factors limit the biogeographic range, so that hypothetical results of climate change may be used to predict effects on populations;

6. The role of mass transport in maintaining or dispersing populations and aggregations, in both the vertical and horizontal.

#### 3.3.5 Historical database

Although there are not as many historical data for copepods in the Northwest Atlantic as exist for fish, some data are available. These should be "mined" and used for hindcasting where possible, as well as for analysis of historical variance, which could be used to assess the magnitude of signal required to identify effects of climate change. Most of the same questions posed for fish populations (Section 3.2.5) are appropriate here.

#### 3.3.6 Modeling

It is recommended that specific modeling studies include:

1. Improvements in understanding of allometric relationships, which would better define the limits of technology which generates data on size-frequency distributions (e.g., acoustics, optics);

2. Development of coupled biological-physical numerical models for copepod populations in the Georges Bank study area and appropriate subregions.

#### 3.3.7 Technology

Certain developments in technology recommended under the GLOBEC program will be applicable to all taxonomic categories of interest, particularly in the case of field sampling instruments. These general recommendations for new instrumentation are discussed in Section 5. Here we recommend the following advances in technology which would be of special value for studies of zooplankton:

1. Improved biotechnological methods for assessing growth rates;

2. Development of moored net sampling devices;

3. Development of image analysis systems or molecular genetics techniques which would enable identification of species and developmental stages;

4. Advent of new biochemical techniques which permit rapid measure of physiological state, such as general locomotory capability, reproduction, and entrance into and exit from diapause;

5. Innovative methods for measuring mortality;

6. Development of better, more freely available, calibrated acoustics hardware.

### 3.4 Benthos

#### 3.4.1 Target species

There is some merit to developing a GLOBEC study around the problem of regulation of abundance and distribution of the sea scallop, Placopecten magellanicus. This could complement studies on Placopecten production and settlement dynamics planned for the Canadian OPEN program. In addition, there is a good historical data base from the fishery for this species. However, the funds available for benthic research in the Northwest Atlantic might better be spent in two alternative ways: (1) to develop a better understanding of the role of physical processes in the ecology, and success of, various key types of meroplanktonic larvae; and (2) to investigate the role of the benthic habitat, both as substrate and as a food supply, for the target demersal fishes.

The rationale for erecting these priorities is as follows. There is concern about whether a sufficiently representative cohort of sea scallop larvae could be well enough tracked in space and over sufficient time to assess accurately how physical processes played a role in dictating eventual abundance and distribution of settlers. That dampens enthusiasm for an exclusive sea scallop population focus to the benthic component. On the other hand, benthic population ecologists almost universally acknowledge that the most critical gap in our understanding of benthic population dynamics is the dearth of knowledge of the ecology of larval life stages of benthic forms. The Northwest Atlantic study affords an important opportunity to erect and test critical hypotheses concerning the ways in which physical processes contribute to the behavior, success, and fate of meroplanktonic larvae as a function of various fundamental differences in larval types. In addition, the understanding of population regulation of the target fish, the codfish, would be grossly incomplete without a benthic study of how the bottom habitat and food availability influence growth, survival, and success of cohorts of codfish. Such a study should in addition evaluate how cod and alternative ground fish, especially the elasmobranchs, interact through mutual exploitation of benthic food and overlapping benthic habitat preferences.

#### 3.4.2 Definable populations

The sea scallop has a well-defined population on Georges Bank, with stock assessment work done annually to update that knowledge. For any study of sea scallops, the Georges Bank population could probably be viewed as reasonably closed and adequately defined.

The recommended study of how benthic invertebrate larvae respond and are influenced by a suite of potentially important physical processes can be assessed without explicit identification of adult populations. These studies probably require only that relatively discrete cohorts of larvae of different types be followed and their behavior, survivorship, and ecology be compared:

1. holoplankton vs. meroplankton;

2. feeding vs. non-feeding larvae;

3. phytoplanktivorous vs. zooplanktivorous larvae;

4. larvae of long vs. short planktonic periods such as 6 vs. 3 weeks;

5. spring vs. fall spawners;

6. strong vs. weak swimmers;

7. large vs. small larvae.

#### 3.4.3 Population dynamics

A modest effort should be devoted to the study of population dynamics of sea scallops, focusing intensively upon larval dynamics. This should be coordinated with the studies of scallop settlement, production, and early juvenile survival conducted as part of the Canadian OPEN program. Because the scallop spawning is much later in the season than the pulse of Calanus, a proper study of cohorts of sea scallop larvae would require continuation of spatially and temporally dense sampling later into the year.

Studies of population dynamics of other benthic invertebrates should be considered if a good candidate species is identified. The ideal target species would have a well-defined adult distribution, spawn heavily and synchronously, and possess easily identified larvae.

#### 3.4.4 Focus on process and mechanisms

Studies should be conducted to assess how fundamental parameters of population dynamics, such as reproduction, growth, larval dispersal, behavior, growth, settlement, and survival, vary directly and indirectly as a function of physical and biological forcing:

1. fronts of different types;

2. stratification of the water column;

3. advective regime;

4. tides;

5. storms;

6. wind field;

7. temperature change;

8. freshwater inputs;

9. primary (food) production; and

10. effects of predators and competitors.

Such studies should be plausibly chosen to be able to yield information useful in predicting the potential effects of climate change. Research projects should address the role of changes in forcing functions (e.g., temperature, turbulence, advection, food, predators, competition, substrate type) on physiological, behavioral, and population responses (e.g., growth, feeding, migration, predation, and settlement site selection).

#### 3.4.5 Historical database

There exist fewer historical data on larval scallops, crabs, and non-exploited meroplanktonic forms than for either zooplankton or fishes. Planktonic samples may be available from previous Georges Bank studies: the potential to exploit these sources should be explored.

#### 3.4.6 Modeling

Numerous modeling studies on either the meroplanktonic stages alone or on the full life history of target benthic invertebrates are encouraged.

1. Data that demonstrate how various physical processes cause differential responses of larva with different life history and ecological characteristics should be used to test and develop new models to explain the selective basis for various patterns of reproduction among benthic invertebrates (e.g., patterns with depth, latitude, body size, habitat, etc.);

2. For the sea scallop cohorts, bio-physical coupling models should be developed to explain and predict the causes of spatial and temporal variation in recruitment of scallop year-classes;

3. Ecosystem models of planktonic communities that include larva of benthic forms of different types are necessary to understand how selective pressures differ as a function of varying life history traits of larvae. These should include evaluation of the role of potential food limitation and predation and how these biological factors themselves are influenced by changing physics;

4. Present models of feeding of benthic animals are extremely crude and need further development and testing;

5. More progress can be expected on the role of physical vs. biological factors in dictating patterns of habitat selection by settling larvae of benthic invertebrates.

#### 3.4.7 Technology

Certain developments in technology recommended under the GLOBEC program will be applicable to all taxonomic categories of interest, particularly in the case of field sampling instruments. These general recommendations for new instrumentation are discussed in Section 5. Here we recommend the following advances in technology which would be of special value for studies of meroplankton:

1. Development of devices to identify and quantify larvae of selected species, perhaps using immunology or other biotechnological tools; and

2. Development of an automated larval sampler which could be deployed on a mooring.