46 



Fishery Bulletin 89(1), 1991 



1965-76 broods show some tendency 

 for the rate of increase in length to 

 slow in the fall, near the end of the 

 fishing season, at ages 17-19 and 27- 

 30 months. The curves for the 1977-86 

 broods differ somewhat in that this 

 slowing of growth in the fall is less pro- 

 nounced. These data suggest that den- 

 sity-dependent growth in pink shrimp 

 may continue into the adult stage. 



The impact that the trawl fishery 

 has had upon the shrimp population is 

 very evident from an examination of 

 age composition of the catch (Fig. 8). 

 In catch years 1966-78, age-3 shrimp 

 comprised an average 20.4% of the 

 catch by number, falling to an average 

 4.9% for the years 1979-88. Converse- 

 ly, age-1 shrimp have risen from an 

 average 30.6% in the early period to 

 69.2% of the catch in recent years. The 

 increases in relative abundance of 

 age-1 shrimp may be explained in part 

 by the fact that they are recruited to 

 the trawl gear earlier in the season due 

 to increased size at age in recent years. 

 The decline in absolute numbers of age-3 shrimp (Fig. 

 8) cannot be explained by changes in gear selection. 

 The observed changes in age structure of the catch 

 must be at least partly due to the impact of the trawl 

 fishery. 



The change in age composition of the catch is also 

 reflected in the mean size of shrimp in the landed catch. 



The average number of shrimp per pound increased 

 from 109.4 during the 1966-78 period to 118.9 for the 

 1979-88 period. The decline in numbers of older shrimp 

 has been accompanied by an increase in the percentage 

 of shrimp maturing directly into females at age-1 (Fig. 

 9). In recent years, levels of primary females as high 

 as 30-50% are common. This effect has compensated 

 for the higher cumulative harvest rates on age-2 and 



AGE ONE GROWTH INDEX 

 CPUE 



Figure 6 



Age-1 growth index (mm) and pink shrimp catch-per-unit- 

 effort (kg/single-rig equivalent hour) for 1966-88. 



