Stock Structure of the Cod Fisheries

Stock Structure of the Cod Fisheries

Introduction

Fisheries management deals with populations or "stocks", usually in reference to geography. A stock is a population or a portion of a population, all members of which are characterised by similarities that are not heritable, but are induced by the environment. A stock may or may not include members of several different sub-populations. Subpopulations are a fraction of a population that is itself genetically self-sustaining. It is the smallest natural self-perpetuating unit. Although differences between sub-populations may be small they are heritable (Iverson, 1996). Members of a subpopulation segregate at spawning time, whereas members of a stock need not.

Effective management and resistance to exploitation, depend partly on how well these identified divisions reflect the lives and behaviour of fish. The fish in different stocks may differ in their size and growth rate, in the range they occupy, in their habits of migration and spawning, or in a variety of other features of physiology or behaviour. There is evidence that many traditionally identified cod stocks contain a number of sub-stocks oriented to particular feeding and spawning grounds along the coast or offshore, despite this, at present the northern cod stocks are managed as one unit (Ryan, 1996).

The genetic makeup of fish stocks is becoming increasingly important in fisheries management. Each individual of any species carries a set of complicated biological instructions in every cell of its body carried in the cell nucleus, in "genes". They govern how each cell functions and how the animal itself develops from a fertilized egg to a mature individual. The cells of most animals have thousands of genes, half of them inherited from each parent. Related individuals share many of the same genes, and commonly lack some of the genes shared by another population of the same species. Patterns like this mean it may be possible to use genetic analysis to distinguish one stock of fish from another, to estimate the frequency of interbreeding between neighbouring stocks, and to find clues to the ancestry of different stocks (Ryan, 1996).

This section gives a brief introduction to the Gadidae family, before focusing on and describing the range of the Atlantic Cod (Gadus morhua). This will be followed by the traditional and genetic methods of establishing and tracing cod stocks and examples of research on stock structure using these different methods. The section will conclude with an evaluation of the two methods.

The Gadidae Family

Table 1: Gadidae Latin and common names

Gadidae

(Cods and Haddocks)

Scientific Name English Name

Arctogadus borisovi

East Siberian cod

Arctogadus glacialis

Arctic cod

Boreogadus saida

Polar cod

Eleginus gracilis

Saffron cod

Eleginus nawaga

Navaga

Gadiculus argenteus argenteus

Silvery cod

Gadiculus argenteus thori

Silvery pout

Gadus macrocephalus

Pacific cod

Gadus morhua

Atlantic cod

Gadus ogac

Greenland cod

Melanogrammus aeglefinus

Haddock

Merlangius merlangus

Whiting

Microgadus proximus

Pacific tomcod

Microgadus tomcod

Atlantic tomcod

Micromesistius australis

Southern blue whiting

Micromesistius poutassou

Blue whiting

Pollachius pollachius

Pollack

Pollachius virens

Pollock

Raniceps raninus

Tadpole fish

Theragra chalcogramma

Alaska pollock

Theragra finnmarchica

Norwegian pollock

Trisopterus esmarkii

Norway pout

Trisopterus luscus

Pouting

Trisopterus minutus

Poor cod

Cod is the common name for nearly 60 species of valuable food fish from the 10 families of the order Gadiformes. The family Gadidae (Fig 1 for taxonomic information) are the true cods (Jordan, 2002) but other families in the order are also known as cod, such as the deep-sea cod of the family Moridae . The Gadidae differ from all other cod-like fishes by the following combination of features; separate caudal fin, dorsal fin is divided into two or three sections, fins without spines, swimbladder physoclistious (no pneumatic duct present), and chin barbell is usually present (Moyle & Chec, 2000). The family of Gadidae consists of 22 nominal species (Table 1). Most members are confined to a relatively high salinity, although a few members tolerate fresh water (Boreogadus saida, Microgadus tomcod & Gadus morhua). Differences within the family include variances in body size (15cm - 2m), geographic distribution, diet and tendency to school and migrate long distances.

Current Range

Most members of the Gadidae inhabit the continental shelves in the North Atlantic with only a few exceptions. Eleginus gracilis, Microgadus proximus and Theragra calcorgramma, which are distributed in the Pacific (Cohen et al,