Seasonal Differences in Spawning, Egg Size, and Early 



Development Time of the Hawaiian Anchovy or Nehu, 



Encrasicholina purpurea^ 



Thomas A. Clarke 



ABSTRACT: Nehu spawning is concentrated in a 1 

 hour period shortly after sunset: the delay after 

 sunset is longer in the winter than in summer. 

 Incubation time for nehu eggs is 22-35 hours and is 

 inversely related to temperature. Development time 

 between hatching and first feeding shows relatively 

 greater seasonal differences, and total embryonic 

 development time during the coldest months is al- 

 most twice that of the warmest months. Nehu egg 

 size is inversely related to temperature. The sea- 

 sonal differences in egg size are probably the result 

 of a physiological response to temperature. The 

 potential adaptive value of the seasonal change in 

 egg size in this tropical species is more likely related 

 to size-specific differences in predation rates rather 

 than to seasonal changes in abundance or size of 

 food for lar\ae. The seasonal changes in total devel- 

 opment time result in marked differences in the time 

 of the diel cycle at which larvae reach first-feeding 

 status; these differences could have more influence 

 on survival of small larvae than effects related to 

 either predation or food availability. 



Seasonal differences in egg size, incubation, and 

 posthatch embryonic development time have 

 been reported for many species of temperate or 

 higher latitude fishes. Some reports, e.g., 

 Blaxter and Hempel (1963), involve differences 

 between stocks or populations with different 

 spawning seasons. Other examples, e.g.. Ware 

 (1977), deal with differences occurring over the 

 spawning season of an apparent single stock. 

 Hypotheses presented about the potential mech- 

 anisms or adaptive significance of seasonal dif- 

 ferences are related to the rather marked 

 seasonal changes in temperature, productivity, 

 etc., that are typical of high latitude environ- 

 ments. Seasonal differences in egg size and early 

 development have not been investigated in 

 fishes from tropical latitudes where seasonal 

 environmental changes are less extreme than at 

 higher latitudes. 



'Contribution No. 790 of the Hawaii Institute of Marine 

 Biology. 



Thomas A. Clarke, Department of Oceanography and 

 Hawaii Institute of Marine Biology, University of Hawaii, 

 Honolulu, HI 06822. 



The Hawaiian anchovy or nehu, Encrasicho- 

 lina puiijurea, spawns throughout the year in 

 enclosed, semiestuarine areas of Hawaii (Tester, 

 1955; Clarke 1987). Preliminary studies indi- 

 cated that spawning occurs over a very short 

 period after dusk. Because of this, it was possi- 

 ble not only to obtain freshly spawned eggs 

 readily, but also to identify "day-classes" of eggs 

 and prefeeding larvae throughout the diel cycle 

 and thus to estimate incubation and development 

 times from field samples. This paper reports on 

 seasonal differences in spawning, egg size and 

 development and considers hypotheses based on 

 studies from higher latitudes in the context of 

 seasonal environmental changes in the tropics. 



MATERIALS AND METHODS 



All material was collected from Kaneohe Bay, 

 a semienclosed basin on the northeast side of the 

 island of Oahu, HI. Plankton samples were col- 

 lected with aim diameter, 5 m long conical 

 plankton net of 0.335 mm mesh. The net was 

 rigged with a three-point bridle to which a ca. 3 

 kg weight was attached. The net was simply 

 dropped mouth downward, allowed to fish to the 

 bottom (12-15 m), and retrieved by a tether at- 

 tached to a choke line about 1 m behind the 

 mouth. The sample was immediatly preserved in 

 a ca. 4% formaldehyde/seawater solution. The 

 present study was conducted during the course 

 of a long-term survey of nehu egg abundance 

 that sampled stations throughout Kaneohe Bay 

 between the hours of 0600 and 1100 at approxi- 

 mately weekly intervals. These samples pro- 

 vided eggs and larvae from the morning hours 

 and also indicated periods and locations of high 

 egg or larval abundance for sampling at other 

 times of the day. 



Previous studies (Clarke 1987) on adult female 

 nehu indicated that spawning occurs during a 

 short period (a few hours) after sunset. In order 

 to determine spawning time more precisely, 

 plankton samples were taken at hourly intervals 

 between sunset and midnight at four different 

 times of the year. Based on data from the most 



Manuscript accepted March 1989. 

 Fishery Bulletin, U.S. 87:593-600. 



593 



