15 



14 



13 



12 



11 



10 



9 



8 



7 



6 



5 



4 



3 



2 



1 







THREE-SPINED 



1 1 1 1 1 1 1 1 — 



30 35 40 45 50 55 60 65 70 



LENGTH 



75 



Fig. 6. Length -frequency distribution by 1-mm incremenLs of threespine and biackspotted sticklebacks impinged at 

 MNPS Unit 2 from 1982 through 1987. 



spring, both species move into salt marshes and 

 tidal rivers to spawn (Worgan and Fitzgerald 

 1981) and both adults and young-of-the-year re- 

 main in the spawning areas until late summer 

 (Fitzgerald 1983). These spawning habits proba- 

 bly accounted for the low catch of these species 

 during the summer. The number of sticklebacks 

 impinged annually at Unit 2 ranged from 16,329 

 in 1983 to 880 in 1985 (Table 2). Approximately 

 32% of all sticklebacks impinged there were taken 

 in 1983 and there has been a decrease in 

 impingement since the mid- 1 980s to levels similar 

 to the mid and late 1970s. The impact of 

 impingement on these two species at MNPS has 

 been mitigated with the addition of sluiceways at 

 Units 1 and 3 and high survival estimates for 

 returned fish of over 90% and 70%, respectively 

 (NUSCO 1988). 



Atlantic tomcod 



The Atlantic tomcod is the most abundant 

 member of the cod family collected in the MNPS 

 monitoring programs. It ranges along the Atlantic 

 coast of North America from Newfoundland to 

 Virginia (Bigelow and Schroeder 1953). 



The Atlantic tomcod was caught in all sampling 

 programs, but was only abundant in impingement 

 and trawl samples. Eggs are adhesive and found 

 attached to the substrate and larvae tend to remain 

 in brackish water spawning areas (Howe 1971), 

 habitats not sampled by the monitoring programs. 

 Tomcod reach sexual maturity at about 130 mm 

 (Howe 1971) and over 98% of those impinged 

 were adults larger than 130 mm (Fig. 7). About 

 90% of those impinged were taken in the fall and 

 winter, during the time of their spawning migration 

 (Howe 1971; Klauda et al. 1987). Over 40% of 

 the total impingement at Unit 2 since 1976 oc- 

 curred in 1982 (Table 2). A marked decrease 



Fish Ecology Studies 273 



