Hunt et al.: Movement of Gadus morhua in the Gulf of Maine area 



855 



individuals. Estimates of growth rates from 

 this study are consistent with the more ex- 

 tensive study on cod growth completed by 

 Shackelletal. (1997). 



Discussion 



Fisheries management objectives for either 

 conservation or yield from a defined manage- 

 ment unit are usually based on a closed sys- 

 tem approach that requires that emigration 

 and immigration of fish from adjacent stocks 

 be sufficiently low so as to avoid confounding 

 stock-status evaluations. Gulland's (1983) 

 operational definition of a stock also consid- 

 ered interchange between stocks and he sug- 

 gested that it can be ignored if conclusions on 

 the population dynamics of the stock remain 

 valid. Therefore, two problems require evalu- 

 ation in assessing stock definitions: 1) Is there 

 evidence of interchange offish at a geographic 

 scale larger than the one used to define a stock 

 boundary? and 2) If interchange does occur, 

 what are the impacts on population dynamics 

 and on the assumptions used in models to es- 

 timate population parameters? 



Tagging experiments can address the first 

 of the two questions provided that the study 

 logistics have not compromised the results and 

 that both ongoing fisheries removals and manage- 

 ment activities during the recapture period do not 

 invalidate interpretation of the results. Capture, tag- 

 ging, and release of cod with bottom trawl methods 

 and T-bar tags can generate a large number of re- 

 leases and, according to a relatively high recapture 

 rate and extended time at large, appear to result in 

 nominal tag- or stress-induced mortality. During the 

 1994 experiment, cod tagged from one tow were re- 

 captured in a subsequent tow, either the same day 

 or several days later and thus may indicate that 

 tagged cod resume a normal schooling pattern shortly 

 after release. Efficiency of the tagging operation is 

 also a contributing factor as is the time of year, depth, 

 and condition of tagged fish. Clay et al. (1989) re- 

 ported on tagging experiments under winter condi- 

 tions and concluded that cod at this time of year have 

 a better chance of surviving the tagging process. Tag 

 loss was not examined in our study but Saunders et 

 al. (1990), in a study of sablefish tagged with tags 

 similar to those used in our study, concluded that 

 tag loss could be as high as 10% in the first year and 

 2% per year thereafter. Less than 10% of cod tagged 

 with T-bar tags for identification in a live-fish hold- 

 ing facility experienced tag loss in the first week, al- 



Figure 8 



Distribution of adjusted cod tag recoveries from releases in the Browns 

 Bank (4Xp) area, aggregated by 10-min latitude and longitude squares. 



though losses were higher after biweekly netting and 

 sampling for length and weight observation (Nelson' ). 



In the present study, the influence of high annual 

 mortality rates (Z), failure to report recaptures, and 

 the effectiveness of advertisements and rewards, the 

 impact of fishery controls, and the potential bias be- 

 tween Canada and U.S. reporting rates may be more 

 substantial than factors such as tag loss. However, 

 these variables are difficult to quantify. 



All cod stocks in the study area have been sub- 

 jected to very high exploitation rates in the last de- 

 cade. For example. Hunt and Buzeta ( 1996 ) reported 

 rates of over 40% in the 1990s for the Georges Bank 

 cod stock. A similar capture rate applied to the tagged 

 population [N] could result in an apparent rate [R] 

 of over 20% returns after accounting for tagging 

 mortality (-20% ) and tag loss (-10%) and assuming 

 full reporting \R = 0.4(7V - A^(0.2 + 0.1) = 0.287V]. 

 Observed return rates were less than half those for 

 the Georges Bank and Browns Bank areas and a lack 

 of reporting could be a contributing factor. Anecdotal 

 information from field staff working in fishing ports 



' Nelson, C. 1996. Department of Fisheries and Oceans, St. 

 Andrews, New Brunswick, Canada. Personal, commun. 



