The Emerging View of New England Cod Stock Structure

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1 Cod Population Structure and New England Fisheries Symposium: Furthering our understanding by integrating knowledge gained through science and fishing Putting it All Together: The Emerging View of New England Cod Stock Structure Steve Cadrin University of Massachusetts, Dartmouth School for Marine Science & Technology

2 The Six Blind Men and the Elephant It s a spear It s a snake It s a fan It s a wall It s a rope It s a tree

3 Interdisciplinary Stock Identification Geographic distribution entire species range and discrete areas Fishing grounds Surveys (juveniles, adults, eggs & larvae) Movement among areas Tagging Egg and larval dispersal Geographic variation among areas Genetics reproductive isolation or adaptation to local environments Phenotypic traits (influenced by genetics and environment): growth rate, reproductive rates, longevity, shape, color, Environmental signals (natural tags): chemistry, parasites,

4 Interdisciplinary Analysis of Cod Stock Structure Previous reviews Traditional analyses (Wise & Jensen 1960, Templeman 1962, Wise 1963) Stock assessment & management units (Serchuk & Wood 1979, Serchuk & Wigley 1992) Recent workshops & investigations (Annala 2012, Zemeckis et al. 2014) Atlantic Cod Stock Structure Working Group ( )

5 Wise & Jensen (1960) Cod Groups eastern Georges Bank, which apparently mixes little with the more westerly and northerly groups although some of the large and older fish do appear to wander off to join the populations of western Nova Scotia (Subarea 4). West of the shoals of Georges Bank (about 68 o W.) there is another population which summers in the great South Channel (about 69 o W.), particularly on the western side, and which spends the better part of the rest of the year inshore in the Nantucket Shoals-Chatham region. North of this group of fish are those of the Gulf of Maine, one or more sedentary stocks. Joining the Nantucket Shoals-Chatham fish in the summer are fish whose winter habitat is outside the Convention Area. They migrate along the Rhode Island, Long Island and New Jersey shores in the autumn, some years reaching as far as North Carolina (about 35 o N)

6 Templeman (1962) Cod stocks in the NW Atlantic

7 Wise (1963) Cod Groups in New England A synthesis of tagging, length composition of catch, parasites, meristics, bathymetry and hydrography suggested four distinct groups off New England: 1. The cod of the offshore banks (Georges and Browns) are closely related to the fish of the southwestern Nova Scotia coast. 2. The cod of the Gulf of Maine, probably divided into many subgroups, and receiving considerable recruitment from the south. 3. The cod of southern New England and the South Channel. 4. The New Jersey coastal cod, which spend part of the year mingled to a greater or lesser degree with the southern New England fish. Cod on Georges Bank are divided from the Great South Channel stock by western shoals of the Bank, at about 68 o W (Wise 1958).

8 U.S. Management Units (Serchuk & Wood 1979) Similar trends in growth rates, survey indices, and commercial catch compositions between Georges Bank and more southern areas, as well as the apparent absence of juvenile cod from the Mid-Atlantic region justified a common approach to assessment and management for both areas. The question of biological stock identity of the southerly cod populations may be moot relative to management concerns. If the similarities between the Southern New England-Middle Atlantic and Georges Bank cod populations result from similar biotic and environmental factors, management measures applied to the two groupings as a unit should produce similar responses within each group. The Gulf of Maine is considered as a separate management unit.

9 US & Canadian Management Units Separate stocks of cod on Georges Bank, in the Gulf of Maine Basin, and on the Scotian Shelf (including one on Browns Bank). Differences in growth and meristics among all three areas Peak spawning on the Scotian Shelf is one month later than that on Georges Bank Tagging, parasites, egg & larval distributions confirm the separateness of these stocks.

10 Canadian Management Units (Hunt 1989) After the US and Canadian boundary decision in 1984, Canada concluded that the Georges Bank cod resource could be partitioned in to two management units: Eastern Georges Bank cod (unit areas 5Zj and 5Zm) The remainder of Div. 5Z and Subarea 6

11 Begg et al. (1999) Compared cod distributions, growth, maturity and mortality among areas off New England Adjacent concentrations on Georges Bank and in the Gulf of Maine, with a division between cod on eastern and western Georges Bank. In most decades, growth rates (K) were different between the Gulf of Maine, western Georges Bank and eastern Georges Bank, with the fastest growth on eastern Georges. In most decades, Georges Bank cod matured younger (age at 50% maturity, A 50 ), with some differences in maturity between eastern as western Georges. Mortality rates were similar among areas. Life history differences support the two current cod stocks and provided some evidence that cod on Georges Bank could be divided into eastern and western components with disjunct distributions across the Bank during autumn.

12 Tallack (2009) Significant growth variation was found between management areas (4X, 5Y, 5Z) and between fine-scale areas (Inshore Gulf of Maine, Georges Bank, Cape Cod and the Bay of Fundy). 5Y 5Z 4X CC IGOM BoF GB

13 2012 Cod Stock Structure Workshop Objectives Review existing data, information, and results of analyses relevant to the stock structure of cod in the Scotian Shelf, Georges Bank, Gulf of Maine, and southern New England regions. Make recommendations on the most likely biological stock structure in these regions (including sub-stock structure if warranted), considering the current management units as a null hypothesis and other stock structure scenarios as alternative hypotheses. Make recommendations for any future research required to evaluate the most likely biological stock structure. Provide advice on the alignment (or mis-alignment) of the current management unit boundaries vis-à-vis the most likely biological stock structure

14 2012 Cod Stock Structure Workshop Conclusions About Broad-Scale Stock Structure Conceptualizing the most likely biological stock structure is essential for the next steps of evaluating alternative management units and whether they are more likely to achieve fishery objectives. Any management unit boundary will be a simplification of a more complex ecosystem, but if the simplification sufficiently reflects reality, we can meet our fishery and conservation objectives (optimum yield, preventing overfishing, etc.). All information from U.S. waters indicates that there are three genetic stocks: 1. Offshore - Eastern Georges Bank (with some connectivity with the Scotian Shelf); 2. Inshore - Northern, Spring-Spawning Complex; and 3. Inshore - Southern, Winter-Spawning Complex. Information from more traditional stock identification information generally supports the genetic perspective (e.g., tagging, growth, larval dispersal). Cod in the eastern Gulf of Maine appear to be distinct from other groups.

15 2012 Cod Stock Structure Workshop Conclusions About Fine-Scale Stock Structure: Larval retention and multi-year fidelity to local spawning sites suggest fine-scale metapopulation structure. Some traditional spawning groups were depleted, and have not been re-colonized by more productive groups. Depletion of historical spawning groups is most apparent in the eastern Gulf of Maine, the Mid-Atlantic, the Plymouth Grounds, and recently Nantucket Shoals. The Workshop agreed that all genetic information available from U.S. waters is not entirely congruent with current management unit boundaries. Many of the workshop participants felt that there was compelling evidence that the current management units need to be revised. The Workshop did not reach any conclusions on what the most appropriate management units might be.

16 Zemeckis et al. (2014)