Wigley and Serchuk: Spatial and temporal distribution of juvenile Gadus morhua 



605 



Discussion 



The observed seasonal variation in distribution of age-3 

 cod relative to age groups 1-2 and 4 + may be associ- 

 ated with a transitional period involving both matura- 

 tion and feeding habits. Age 3 encompasses a period 

 in which there is a mismatch in size-at-first-maturity 

 and the attainment of the adult diet. Median size and 

 age at sexual maturity for cod is about 50cm and 2.5 

 yr, respectively (Livingstone and Dery 1976), and dur- 

 ing spring some age-3 fish recruit to the spawning 

 population. Autumnal co-occurrence of age-3 fish with 

 ages 1-2 may be related to diet. Bowman and Michaels 

 (1984) presented data which indicate that cod < 66-70 

 cm have not assumed the adult diet dominated by fish; 

 the mean length of age-3 cod in autumn is 68.0cm 

 (Table 1). 



In a mathematical evaluation of spatial distributions 

 of several North Sea species, Houghton (1987) sug- 

 gested that cod distributions were more complex and 

 less persistent than those observed for haddock or flat- 

 fish. In this study, however, the spatial and temporal 

 patterns observed for juvenile Atlantic cod from land- 

 ings data were remarkably uniform over the study 

 period, and did not seem to vary according to stock size 

 or year-class strength. The use of commercial data in 

 this study is somewhat constrained by management 

 regulations, fishing practices, and the distribution of 

 fishing effort, yet results from analysis of survey data 

 in this study seem to corroborate these conclusions. 



The use of mean values in this study to define pat- 

 terns of temperature and depth distribution may better 

 reflect general tendencies rather than absolute pref- 

 erences; in actuality, cod of all ages except age were 

 found at virtually all available temperatures and 

 depths. Yet the patterns that emerged in this study are, 

 for the most part, consistent with those identified in 

 other studies. Both Schroeder (1930) and Wise (1962) 

 noted the tendency for older cod to move into greater 

 depths. Scott (1988) found that cod in colder Canadian 

 waters were distributed at temperatures of 2-10° C, 

 with largest catches occurring at 4-6°C. The apparent 

 contradiction posed by the movement of cod to deeper, 

 warmer water in winter-spring observed by Scott 

 (1988), and the observations in this study of movement 

 to shallower water on Nantucket Shoals and Georges 

 Bank during this period, is an artifact of the different 

 temperature regimes for the two regions; in each case, 

 cod are changing depth locations to maintain preferred 

 temperatures. Schroeder (1930) reported cod occur- 

 rences within an annual range of 0-17 °C in the region 

 from Nantucket Shoals to North Carolina, and attrib- 

 uted the triggering of the autumn migration of Nan- 

 tucket Shoals adult cod westward to New Jersey for 



winter spawning to falling bottom temperatures in Oc- 

 tober. Similarly, movements of juvenile cod from Nan- 

 tucket Shoals to deeper water off Chatham and the 

 Great South Channel in summer-early autumn were 

 thought to be in response to locally-available, cooler 

 temperatures (Schroeder 1930). Wise (1958, 1962) 

 determined from tagging studies that a resident pop- 

 ulation of cod inhabited the Nantucket Shoals-Great 

 South Channel area year-round, but that Nantucket 

 Shoals also represented the summer residence for the 

 population of cod that wintered off the coast of New 

 Jersey. Thus, the distribution patterns observed in this 

 study within SAR 521 and 526 would most likely reflect 

 seasonal movements of resident cod, although the 

 migratory population may partially contribute to land- 

 ings for quarters 2 and 3 in SAR 526. 



Scott (1982) concluded from an analysis of fish dis- 

 tribution by bottom type that, although generally asso- 

 ciated with sand-gravel sediments, cod occurred over 

 all substrates and that observed patterns of distribu- 

 tion were more likely due to the bottom-type prefer- 

 ences of major prey items (e.g.. Cancer crabs, sand 

 lance, Ammodytes sp., etc). Although no quantitative 

 analysis of distribution by bottom sediment was under- 

 taken here, the seasonal shifts in concentration iden- 

 tified in this study do not suggest any major change 

 in substrate preference of cod. However, the Great 

 South Channel and the Northern Edge-Northeast Peak 

 regions, where concentrations of scrod cod occur in 

 quarters 3 and 4, are characterized by coarser sediment 

 types than those generally found elsewhere on Georges 

 Bank (Wigley 1961, Schlee 1973). 



Based on the above analyses, there is evidence for 

 well-defined seasonal and geographic shifts in concen- 

 tration for juvenile Atlantic cod in the Georges Bank- 

 Southern New England region. Moreover, these pat- 

 terns of concentration appear to be associated pri- 

 marily with temperature. The high level of spatial and 

 temporal resolution possible, i.e., 10-min squares of 

 latitude and longitude and quarters, suggest that this 

 type of study may be useful in assisting fisheries 

 managers with decisions regarding seasonal and areal 

 closures under the Flexible Area Action System. 



Acl<nowledgments 



We express our thanks to S.L. Brunenmeister for her 

 assistance in the initial stage of this work, J. Burnett 

 and R.K. Mayo of the NEFSC and anonymous review- 

 ers for their helpful comments and review of this 

 manuscript, and N.G. Buxton of the NEFSC for 

 graphic assistance. 



