Fishes of the Western North Atla?it'tc 191 



Determination of growth rates and ages at different lengths after hatching pre- 

 sents a major problem. Obviously the analysis of length-frequency curves is fraught 

 with difficulty in a fish which breeds on a year-round basis, even though the species 

 has peaks of productivity in June— July and November-December. Study of the hard 

 parts by sectioning and clearing techniques is also unrewarding, although examination 

 of whole vertebrae after subjecting them to various treatments shows considerable 

 possibilities for age determination. The return of tagged fish has been too limited to 

 be of use in checking growth estimates, and aquarium fish have not provided any data. 

 Furthermore, we have taken relatively few small individuals in our collections. Piecing 

 together all the fragmentary information at our disposal, it seems probable that this 

 species grows at an average rate of 8-10 cm a year for the first three years following 

 hatching and more slowly thereafter. Thus a 20 cm Skate might be i — 1.5 years old, 

 a 30 cm individual 2—3 years of age, a 40 cm Skate 3-4 years, a 45 cm fish 4-5 and 

 possibly 6 years, and a 50 cm individual 6—7 and perhaps 8 years old. Specimens 

 above 50 cm are comparatively rare and the great majority are males; the largest we 

 have recorded is 54 cm. In monthly samples of Block Island Sound fish the length- 

 frequency curves show the peak of abundance to be between 43 and 46 cm; above this 

 point the curve falls away very steeply, thus indicating a high mortality rate after the 

 fish are approximately five years old. 



In general, female specimens are adolescent in the range between 32 and 43 cm; 

 we have found fish up to 42.5 cm which were still immature, as judged by the shell 

 gland, size of the uterus and condition of the ovaries, and we have found fully mature 

 individuals as small as 36 cm. Males tend to reach adolescence at a somewhat greater 

 size, the general range being from 36 to 45 cm; we have found immature males as large 

 as 44 cm and fully mature males at 37 cm. 



With respect to the average length and weight of adolescent and fully mature 

 males and females, there appear to be significant differences between the Long Island 

 and Block Island Sound populations: 



Adolescent Adolescent Mature Mature 



females males females males 



Long Island 38.4 cm 40.0 cm 42.4 cm 43.1 cm 

 344 g 338 g 475 g 455 g 



Block Island 42.0 cm 42.2 cm 45.8 cm 46.4 cm 

 462 g 422 g 596 g 587 g 



These figures represent yearly averages, and it is abundantly clear from them that 

 either the Long Island Sound fish mature earlier than those from Block Island Sound 

 or they grow more slowly and perhaps mature at approximately the same age as their 

 counterparts to the east. We suspect that the latter alternative is the true situation. 

 Continuing our comparison of samples of trawl-net catches from the two areas, 



