FISHERY BULLETIN: VOL. 85, NO. 3 



as Div. 3K. Precise information on fishing mortal- 

 ity is unavailable. 



The age composition for the 1970-78 period in 

 Div. 3K indicates that little more than 109^ of the 

 population was older than 15 years. However, 

 more than 40^/c of the commercial otter trawl 

 catches in 1976 by both Canada and Poland in 

 Div. 3K were comprised offish 15 years and older, 

 and more than 309f were 17 years and older (Dow- 

 ering and Pitt 1977). Thus the impact of the fish- 

 ery was greatest on the older age groups, and this 

 could explain the rapidity of the disappearance of 

 the population. 



Accompanying the reduced age span was an 

 overall increase in size at age for both sexes in all 

 three divisions. The mean size at age for older fish 

 showed a considerable increase from the earlier to 

 later periods examined, whereas the mean size at 

 age for the younger fish was not very different 

 between the two periods. The substantial increase 

 in size of older, commercially exploited fish may 

 be the result of reduced abundance as indicated 

 by the reduced age span. While there is no direct 

 evidence here of density dependent growth, there 

 have been studies published which show that it 

 does occur. Bowering and Brodie (1984) showed 

 that there was a systematic increase in mean size 

 at age of witch flounder in the Gulf of St. 

 Lawrence from 1976 to 1981 for age groups fully 

 recruited to the commercial fishery accompanied 

 by a significant reduction in the age span of the 

 stock. They suggested it was likely the result of 

 increased exploitation and subsequent reduction 

 in abundance. They also showed that because of 

 the increase in growth rate in particular the stock 

 biomass remained relatively stable despite the 

 fact that the stock abundance had been reduced. 

 Unfortunately, estimates of stock abundance and 

 biomass are not available for the earlier periods 

 of this study for comparison. 



Bowering (1976) ruled out temperature as a 

 major contributing factor for changes in growth of 

 witch flounder in the Canadian Northwest At- 

 lantic for two reasons: 1) they mainly inhabit 

 depths that are not usually subjected to wide fluc- 

 tuations in bottom temperature and 2) the growth 

 rates of witch flounder in the more southerly re- 

 gions are much slower than in the more northerly 

 regions, the opposite of what one would expect if 

 temperature were considered to have a signifi- 

 cant influence on its growth rate. Bowering and 

 Brodie (1984) suggested that given the feeding 

 behaviour of adult witch flounder as described by 

 Rae (1969), competition with other species is un- 



likely to be a significant factor in changes in 

 growth rate, and within species competition is 

 likely to be a more important factor. 



Sexual Maturity 



It should be pointed out that immature males 

 are not particularly well sampled by the survey 

 gear, and, therefore, the maturity rates may be 

 slightly biased. However, any bias would be con- 

 sistent for males in all divisions. On the other 

 hand, although younger females are also not well 

 sampled, the first occurrence of mature fish in the 

 samples is reasonably well established. Most of 

 the data here were collected in late summer and 

 early autumn, several months after spawning, 

 and one could have some concern as to the inter- 

 pretation of immature versus fully recovered 

 gonad condition. I do not feel, however, that witch 

 flounder in these areas studied present signifi- 

 cant cause for concern in this regard. The Green- 

 land halibut, a flatfish whose gonad condition is 

 more difficult to interpret, was sampled from 

 northern Labrador in the autumn for visual inter- 

 pretation of gonad condition complemented by 

 histological analysis by Walsh and Bowering 

 (1981). They found no significant error in the vi- 

 sual (at sea) interpretation of gonad condition, at 

 least for females. 



Due to the time of year when sampling oc- 

 curred, a slight bias in true size and age and ma- 

 turity may occur; however, it should be consistent 

 throughout the division and time periods exam- 

 ined. Therefore, comparisons should not be bi- 

 ased. Despite such concerns, for the data pre- 

 sented here, it would appear that there was a 

 significant reduction in both mean length and 

 mean age at 509^ maturity for both Div. 2J and 3L 

 females and Div. 3L males (for mean age only) 

 over the study period. For Div. 3K there was no 

 significant change in either mean length or mean 

 age at 50% maturity for either males or females 

 over the time period. 



Molander (1925) found that for plaice and 

 flounder in the Baltic, maturity appeared at a 

 lower age but at a higher length with increased 

 growth rate, and suggested that when growth 

 rate was poor, the age at maturity was higher. 

 Pitt (1975) found that for American plaice on the 

 Newfoundland Grand Bank, the faster growing 

 fish matured at an earlier age but at approxi- 

 mately the same size. Bowering and Brodie ( 1 984 ) 

 found similar results for witch flounder in the 

 Gulf of St. Lawrence. It has been suggested. 



628 



