FISHERY BULLETIN: VOL. 70, NO. 3 



LARVAL SURVIVAL AND YEAR-CLASS 

 STRENGTH 



Rothschild (1965) conceded that environ- 

 mental conditions a few months prior to the 

 Hawaiian fishing season affect the availability 

 of skipjack but felt that the large variability in 

 catch rates is due to variability in the year-class 

 strength. The variability in catch rates is also 

 apparent within the two broad categories of 

 years defined earlier in this paper. In the next 

 section it will be sho^vn, however, that the var- 

 iability can be caused by variations in the cur- 

 rent field. Nevertheless, year-class strength is 

 not excluded as an additional cause for the var- 

 iations in catch rates. It was previously noted 

 that the large interyear differences in sea-air in- 

 teraction processes observed during the TWZO 

 investigation would affect productivity (plant 

 production) and, hence, also larval survival. In 

 mid- and high latitudes, winter overturn reg- 

 ularly replenishes the nutrients of the surface 

 layer. In a large portion of the tropical and sub- 

 tropical ocean this cyclical replenishment does 

 not take place and a permanent pycnocline inhib- 

 its vertical exchange of the surface nutrient-de- 

 ficient with the deeper nutrient-rich water. 

 Other than in areas of upwelling such as along 

 the equator and possibly near islands, nutrients 

 enter the surface layer by eddy diffusion. A net 

 heat gain in the tropical and subtropical oceans 

 tends to increase the stability of the pycnocline 

 and thus inhibits the eddy diffusion process. 

 Wind stirring, vertical current shear, and in- 

 ternal waves tend to enhance eddy diffusion. The 

 wind speed also affects the evaporation which, 

 in turn, affects the net heat exchange across the 

 sea surface. The evaporation rate may be so 

 large that there is a net heat loss from the sea 

 surface and convective overturn takes place, in- 

 creasing the nutrient supply of the surface layer. 



Increased vertical diffusion across the pycno- 

 cline due to favorable sea-air interactions may 

 have subtle effects in that it need not be reflected 

 as an increase in nutrient concentration. In 

 areas where nutrients limit productivity, an in- 

 crease in the nutrient supply into the surface 

 layer can be entirely exhausted by an increase 

 in productivity. Thus, a low phosphate concen- 



tration and absence of a seasonal variation in 

 the trade wind region of the North Pacific Ocean 

 does not preclude variations in productivity. 



The sea-air interactions in the trade wind zone 

 from January to April 1964 (Table 4) favor a 

 larger nutrient supply by diffusion and, there- 

 fore, higher productivity than do those for the 

 same months of 1965. Consequently, a better 

 supply of primary producers in 1964 should have 

 enhanced larval survival. The TWZO larval cap- 

 tures in 1964 and 1965 are consistent with this 

 proposition. There is presently no information 

 to verify the hypothetical sequence of events. 



In this discussion the results of the TWZO 

 investigation are used to illustrate what proba- 

 bly takes place throughout the tropical and sub- 

 tropical oceans and, therefore, in all the skipjack 

 spawning areas. The illustration does not imply 

 that the North Equatorial Current or the Ha- 

 waiian waters are major skipjack spawning 

 grounds. 



The sequence of events described is amenable 

 to quantitative study. Productivity models exist, 

 such as the one used by Parsons and Anderson 

 (1970), that can be adapted to reflect the en- 

 vironmental changes of the skipjack spawning 

 areas. An integral part of the productivity 

 studies must be adequate sampling of the animal 

 community, including skipjack larvae, that di- 

 rectly depend on the initial stages of the food 

 chain. Those studies would lead to a recruitment 

 or year-class strength model that complements 

 the drift model. Development of a year-class 

 strength model is not within the scope of this 

 paper. 



A DRIFT MODEL 



The displacement of a fish school was ex- 

 pressed above by 



o = Sw + Sf. 



Here I wish to consider only the contribution to 

 the total displacement of fish schools caused by 

 the currents, Sw, in a portion of ocean between 

 lat 10° and 25°N, and long 120° and 160°W, the 

 model ocean. In this idealized, rectangular ocean 

 the distances between degrees of latitude and 



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