Population Dynamics Working Group Report

Chairman: David H. Cushing
Rapporteur: Pierre Pepin

At the meeting, our information from the primary groups was given informally by the members who happened to be present. With the written reports, the objectives of the three primary groups are stated more clearly. I now write the sort of conclusion we might have come to had our objectives been more clearly stated. I deal with the three primary groups and write a short synthesis.

9.4.1 Fish

Objective To study the changes in growth and numbers of cod and haddock larvae along the 80-100m isobath in May with respect to the vertical stratification. A cohort of larvae will be followed from the spawning ground and will be sampled at frequent intervals.

Method The larvae will be sampled in the whole water column and at frequent intervals of depth. Their growth will be estimated by changes in length, weight and by examination of the daily growth rings on the otolith (if possible). Such studies will be executed in the mixed water column before stratification and above and below the thermocline after it has become established. At the same time, numbers will be sampled above and below the thermocline to estimate survival under conditions of rich and poor food.

The fish larvae feed on Pseudocalanus and later on Calanus. The gut contents of the larvae and their gut evacuation rates should be measured. Calanus can be sampled at all stages with nets, but Pseudocalanus should probably be sampled by pumps or fairly large water bottles (John Nichols of the Lowestoft Laboratory has a paper in press on the mesh selection of plankton nets). It would be desirable to estimate the production of larval food so that larval growth can be matched to the quantity of food eaten.

The mortality of fish larvae will be estimated by Lagrangian drift experiments and by the decline in numbers produced. The real point here is that the latter could be successful if the sampling were very concentrated in space and time. In other words a cruise is not used to produce one observation on numbers, but as many as possible. It would also be desirable to estimate the changes in survival in the mixed water before stratification and below and above the thermocline after it has become established. Finally, we need to know the predators and their effect on the mortality or what are the components of loss, predation, advection and diffusion. I shall deal with the need for physical measurements more explicitly below.

9.4.2 Zooplankton

Objective The zooplankton group will study Calanus finmarchicus from January to June along the 80-100m isobath, in terms of birth, growth and death in the context of physical transport processes on Georges Bank. Particular attention will be paid to sizes, stages and genetic and physiological indicators in the vertical distribution.

Method Each of the monthly cruises should be arranged so that weekly samples of the population can be taken. The vertical distribution of calanids and food will be studied with respect to the onset of stratification. Growth should be studied to include the molting process and we should improve the present accent on exponential growth and on the use of the von Bertalanffy equation. Again, growth should be studied in the mixed water before stratification and both above and below the thermocline. The fecundity of the winter spawners should be studied in situ as well as experimentally. In addition to the proposals given, the rates of mortality should be studied in the evolution of the cohort and the rates obtained should be linked to the numbers of predators. This requires that the predators be properly identified.

9.4.3 Benthos

Objective The group will study the larval ecology of six contrasting groups, particularly in their response to physical features and processes.

Method Benthic spawners (but not scallops) occupy fixed positions and the diffusion of larvae from that area can be described with respect to physical processes. The problem is that larvae from a small patch will spread quickly and become lost. But those from a large patch could be followed till settlement and losses by diffusion and other physical processes to the settled patch could be estimated. The problem is to find the six species with large spawning areas.

Synthesis Let the zooplankton proposal be the core of the program and the other two primary groups fit into that scheme. Then the physical problems are twofold:

  1. To estimate the loss of calanids and fish larvae along their drift from their spawning ground, by advection and diffusion. At the same time the proportion retained might be estimated. Such loss rates due to physical causes should be studied with respect to the analogous loss rates due to predation.
  2. To estimate the growth of calanids and fish larvae in the mixed water before stratification and later above and below the thermocline. Such studies should be linked to the food available, Pseudocalanus, Calanus for the fish larvae and phytoplankton for Calanus. The onset of stratification should be described physically with a view to the construction of a general model.
What is the link with climatic change? It is assumed that the changes in recruitment to the stocks of cod, haddock and of Calanus are rooted in the early stages of the life history. Then we can only understand the links between recruitment and climatic change, if the population parameters in those early stages are described and understood.

There are many proposed studies not dealt with, which however will proceed. We have only picked those which bear on the population parameters which may in the future reveal the effect of climatic change on the cod, haddock, Calanus and some unspecified benthic organisms.

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