| Summary: | Samples collected from the eastern North Atlantic between 10°N and 60°N were used to study the variability in aspects of the biology of the mesopelagic fish genus Cyclothone (Gonostomatidae). Intra-and interspecific differences in the maximum size, size at maturity, sex ratio, spawning period, egg size and fecundity were compared among populations from different latitudes. Eight species were found to inhabit the sampled area. C.braueri and C.microdon had the most extensive ranges at the northern end of the transect and were most suitable for intraspecific comparisons. C.pseudopallida, C.pallida, C.livida, C.acclinidens, C.alba and C.obscura were primarily found at the southern extent of the transect. Populations of C.braueri and C.microdon at low latitudes were generally smaller, reached maturity at a smaller size, spawned earlier in the year and probably spawned over a longer period than those at high latitudes. In both species, fecundity was highest at 40°N in the spring. The number of eggs found in females was lower both at other latitudes and other seasons. The results suggested that C.microdon may not spawn at the northern extent of its range. Some characters, such as sex ratio and pre-spawning egg size, were similar among species. Within the shallow-living species, those at lower latitudes attained a larger maximum size, appeared to be longer lived and had a larger size at maturity. However, C.alba was an exception to these general patterns. Deep-living species showed similar trends, however maximum size did not appear to differ among species. Observed fecundity was lower among species at lower latitudes. Standard length and latitude were the best predictors of fecundity. Depth only indirectly affected fecundity in that larger individuals and species were usually found at greater depth.
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