SECKEL: SKIPJACK AND ENVIRONMENT 



gradient separating the North Pacific Central 

 water from the California Current Extension 

 water in 1964 (Figure 4) coincided with a nor- 

 mal fishing year, a sharp gradient in 1965 coin- 

 cided with an exceptional year. The character 

 of the gradient is determined by the currents. 



The importance of currents in marine biology 

 is recognized. Examples of how currents af- 

 fect marine life have been given by Laevastu and 

 Hela (1970). However, the active part that 

 ocean currents play in the migration of adult 

 fish has not been stressed. The role of currents 

 in fish migrations including the movements of 

 fish without reference to landmarks has been 

 described by Harden Jones (1968) . The role has 

 been covered only qualitatively, however, and 

 tuna migrations were not amongst his examples. 



The destination and migration path of a fish 

 is the vector sum of the water velocity and the 

 swimming velocity relative to the water. The 

 contribution of the currents to the migration of 

 skipjack that enter the North Equatorial Cur- 

 rent in the eastern Pacific has been examined 

 by means of a simple drift model. Essential in 

 this model are the geostrophic flow of the North 

 Equatorial Current and a northward component 

 of surface wind-driven current resulting from 

 the trades. In this current system floating ob- 

 jects or fish schools uniformly distributed be- 

 tween lat 10° and 20°N at long 120°W would be 

 concentrated by the meridional component of 

 wind-driven flow as they drift westward. 



This concentrating mechanism is evident in 

 Figure 13 where the southern boundary of the 

 drifting objects (fish schools) shifts northward 

 with increasing west longitude. Additionally, 

 objects (fish schools) are concentrated where a 

 southerly component of geostrophic flow and a 

 northerly component of wind drift converge 

 north of lat 20°N (Figure 13). The minimum 

 time required to reach the Hawaiian Islands 

 ranges from 21 to 23 months (Table 7). The 

 computed drift time is of the same magnitude as 

 the time of freedom of tagged fish (Table 5), 



Before examining the consequences of this re- 

 sult, it is useful to place some limits on the nav- 

 igational abilities of skipjack. (See also Hard- 

 en Jones, 1968.) In terms of physics, it is hard 

 to understand how, in the open ocean without 



a fixed reference, a fish knows that he is in a cur- 

 rent. Only in accelerating flow would he be able 

 to feel a force. The fish, therefore, does not know 

 whether he is swimming with or against the 

 current. 



Easier to understand is the ability of a fish 

 to swim in the direction of his choice. He may 

 also know from the water properties, the type 

 of forage, or from celestial navigation, that he 

 is not in the area of his choice and therefore 

 may set a course for a more desirable environ- 

 ment. Even in this eventuality, his destination 

 is aff'ected by the current. 



Thus, the distribution of skipjack, whether 

 they swim randomly and drift with the current 

 or swim in a predetermined direction, is affected 

 by the northward component of the wind-driven 

 current and the convergence near the northern 

 edge of the Equatorial Current. 



As a result of the numerical model it can be 

 postulated that a possible, and the simplest, mode 

 by which skipjack travel from the eastern Pa- 

 cific to Hawaii, is by swimming randomly and 

 drifting with the current. This mode of travel 

 is consistent with the empirical associations that 

 were described and does not contradict the ap- 

 plicable portion of Rothschild's (1965) migra- 

 tion model. Rothschild statistically related the 

 time of warming in Hawaiian waters with an- 

 nual landings and stated "... that 44 percent of 

 the variation in catch is accounted for by time of 

 warming, the other 56 percent being unex- 

 plained." This statement can be misinterpreted 

 in that it implies a causal relation between time 

 of warming and catch rates. Rothschild appar- 

 ently wishes to demonstrate that there is a varia- 

 tion in catch rates that is not associated with the 

 variation of time of warming. This conclusion is 

 also evident from Figure 2 which shows a rel- 

 atively large range of catch rates within the ex- 

 ceptional and normal types of years. Rothschild 

 examined the differences and their causes in the 

 size frequency distributions of the eastern North 

 Pacific and Hawaiian skipjack fisheries. He con- 

 cluded "... that year-class associated phenomena 

 play an important role in controlling the abund- 

 ance of skipjack in Hawaiian waters." Roth- 

 schild, however, neglected to consider the effects 

 of currents on the distribution of skipjack. 



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