NWA - Chapter 8.2

## Northern Cod

### by Jake Rice

This stock, defined as those cod which inhabit NAFO Division 2J, 3K and 3L, has supported fisheries since at least the 1500s and probably earlier. It is generally agreed that some exchanges occur between Divisions 2J and 2H in the north and between Divisions 3L and 3N0 in the southern extremes of the stock. However, it is assumed that these exchanges balance in some way and may be fairly consistent from year to year. Based on export records, estimates of harvest levels have been developed going back nearly 150 years (Fig. 8-1). These records may not be completely accurate but warrant analyses due to the length of the series. Year to year variation is large, but trends on multiyear scales appear to be present; for example landing estimates in the 1890s appear lower than in the 1880s or 1900s. Overall, landings seemed to vary around or a bit above 200,000 t per year. With the development of the highly mechanized offshore trawler fisheries late in the 1950s, landings rose rapidly to over 600,000 t, and were at that level for most of the 1960s. Late that decade the stock collapsed, and despite intensified fishing effort, landings fell dramatically.

During this period more thorough biological sampling and reporting was instituted, so full age structured population estimates became possible. These estimates show clearly the collapse of the stock, reaching a nadir in 1975-76, and commencing rebuilding in the late 1970s (Fig. 8-2). The age 3+ biomass of cod for the Div. 2J3KL stock was just over 2.5 million tons during the early 1960s. Large amounts of fishing effort by foreign fleets caused the stock to decline and by the mid-1970s the population biomass had shrunk to an all-time low of about 500,000 t. When Canada extended its fisheries jurisdiction in 1977 and dramatically reduced the amount of fishing effort, the stock responded and by 1984 had increased to about 1.2 million tons. Since 1984 the biomass had declined slightly because of the size of the extremely weak 1983 and 1984 year-classes. The age 3+ biomass for 1989 is estimated to be about 800,000 t.

With reduced fishing mortality the stock rebuilt until the mid 1980s. By 1985 fishing mortalities had crept up to the range of 0.4 to 0.5, and the stock stabilized in size. The 1983 and 1984 year-classes were much weaker than the recent average recruitment ( 150 million fish at age 3, compared to an average of around 300 million), so the biomass of the stock has actually declined by around 10 % in 1989 and 1990. The 1986 and 1987 year-classes appear much stronger than average, however, and if harvest levels do not exceed the recent TACs (in the neighborhood of 200,000 t), further growth of the stock is expected.

It is generally believed that cod of the Div. 2J3KL management unit are comprised of a number of somewhat discrete subgroups that gather for spawning on the shoreward slopes of offshore banks between April and June. There may be many spawning components, some of which have been defined: Hamilton Bank, Belle Isle Bank, Northern Funk Island Bank, Southern Funk Island Bank, North Cape of the Grand Bank, and Woodfall Bank. While a large portion of the stock is distributed on these offshore banks during spawning, after that time large quantities of post-spawners move to inshore areas during the early summer to feed on capelin which have aggregated at the coast to spawn. It is during this time when cod are in inshore areas that a large degree of intermingling of the cod from the discrete offshore spawning components occurs.

Water movement over the continental shelves of Div. 2J3KL is generally southward. The Labrador Current, the largest influence on this southward movement, transports some of the coldest surface water in the North Atlantic. The vertical structure of 2J3KL water is comprised of three layers. The upper layer which extends to about 40-50 m has temperatures in the warmest months reaching 10 - 12°C. The cold intermediate layer extends to depths of about 150-200 m with temperatures as low as -1.8°C. The warmer bottom layer is influenced by waters from deeper more oceanic areas.

There is a public perception that the Northern Cod stock has actually collapsed seriously in the mid-late 1980s. This erroneous perception exists because the assessment estimates of the size of the stock did drop greatly from 1987 to 1989. This change arose from a re-evaluation of the research vessel data; decreasing confidence in the commercial catch-per-unit-effort, because of technological changes in the fishing fleet; and improvements in the sequential population analysis methods. The changed view of the stock applies to the entire rebuilding trajectory of the stock, however; the stock did not drop; rather the retrospective estimates all changed. Many of the reasons for the dramatic change in perception of the stock status arise from limitations on understanding how availability of fish to commercial and research gears is influenced by oceanographic conditions. Moreover, expectations of strong rebuilding prevailed in all clients, because of projections made in the mid 1970s, using recruitment estimates from the early years in Fig. 8-2. There are serious reservations about the reliability of many of the reported data from the early 1960s, but even with those uncertainties, the stock appears to have been much more productive in the 1960s than in the 1970s and 1980s. Both the oceanographic influences on availability and the changes in productivity make it vital to improve our understanding of environment-ecosystem interactions. Fisheries oceanography and predator-prey dynamics are major components of the $43 million Northern Cod Science Program. These initiatives provide a natural foundation for development of strong linkages to research programs on marine aspects of climate change nationally, continentally, and globally. Such linkages will be pursued aggressively.