Latitudinal Growth Variation in Young Tautogs: A Field, Laboratory, and Growing Degree‐Day Assessment of Potential Intrinsic Differences in Growth Responses to Temperature

Abstract

A difference in size at age of young Tautogs Tautoga onitis has been reported from two sites in the Mid‐Atlantic Bight, USA, whereby southern fish (Virginia [VA]) reach a twofold greater size than northern fish (Rhode Island [RI]) by age 1 but grow at similar size‐dependent annual rates thereafter. We investigated the basis for this difference by using a coordinated field, laboratory, and growing degree‐day (GDD) approach. Age‐0 Tautogs were sampled from three locations (Chesapeake Bay, VA; Delaware Bay, Delaware [DE]; and Narragansett Bay, RI) and showed a clinal relationship of decreasing fish size and growth rate with increasing latitude. Otolith‐based estimates of larval stage duration and growth rate showed no significant clinal relationship. Young Tautogs from these locations were reared in common‐garden experiments (at 5, 15, and 25°C) to determine whether there were intrinsic postsettlement growth differences related to temperature. None of the location‐specific groups of age‐0 Tautogs exhibited growth that was consistently faster than that of Tautogs from the other locations over all temperatures examined. These data and the results of a GDD assessment between VA and RI indicate that latitudinal differences in temperature regime and growing season length are the major factors determining latitudinal growth differences in age‐0 fish. However, age‐1 RI fish of equivalent size grew significantly faster than age‐0 RI and DE fish and similar to age‐0 VA fish at 15°C and grew 40% faster than all age‐0 fish at 25°C, suggesting countergradient growth variation in age‐1 and older northern Tautogs. Size‐selective overwinter mortality of age‐0 northern fish—whereby smaller individuals are most susceptible to low temperature—is proposed as the likely underlying mechanism leading to faster growth of older northern fish at temperature. Such a mechanism would at least partially compensate for the lower temperature and shorter growing season at higher latitudes.