DISTRIBUTION OF EGGS AND LARVAE OF JACK MACKEREL 



249 



1956: p. 8 and 12). The catch fluctuations since 

 1947 arise primarily from three causes: (1) Avail- 

 ability of sardines and Pacific mackerel, (2) fluc- 

 tuations in demand for canneil jack mackerel, and 

 (3) availability of tlie jack mackerel. Since jack 

 mackerel are packed as "substitute sardines," 

 catch data do not necessarily reflect the size of the 

 adult population. Eventual independence of the 

 industry is assured, however, by the world's in- 

 creasing need for cheap protein and bj^ techno- 

 logical advances in the packing of this species. 



Jack mackerel are taken with a variety of gear; 

 however, more than 99 percent are taken with 

 purse seines and lampara nets. The operation of 

 this gear is described by Scofield (1951). Some 

 jack mackerel are taken b}' sport fishermen using 

 live bait. In 1953, the sport catch was unusually 

 large, nearly 200,000 fish being taken (Fitch, 1956: 

 p. 27). However, this catch amounts to less than 

 2 percent of the commercial catch. 



E.xcept for a minor amount used in the fresh-fish 

 market, the commercial catch is used for canning. 

 Jack mackerel are principally packed sardine style, 

 usually in tall, 1 pound cans. A small part of the 

 catch is packed in other ways. 



To date, the catch of jack mackerel has not un- 

 dergone any sustained decline. Therefore, all the 

 previously mentioned data take on an added signif- 

 icance when one considers that manv fishery in- 

 vestigations are initiated after the industry has 

 experienced a decline in the number of catchable 

 fish (Walford, 1948). By noting the variations in 

 the strength of spawning, stock size, mortahty, 

 et cetera, now while fishing mortality is relatively 

 low, future observations under conditions of higher 

 fishing mortality should permit the assessment of 

 the effects of man on the population size of the jack 

 mackerel. 



METHODS OF COLLECTING DATA 



Since this study of jack mackerel constitutes but 

 part of a larger and more comprehensive studv of 

 the ecology of pelagic fishes off the coast of Cali- 

 fornia, the methods used are those originated by 

 the Bureau of Commercial Fisheries Biological 

 Laboratory at La Jolla and adopted by the stafl"s 

 of the California Cooperative Oceanic Fisheries 

 Investigations. These methods iiave been planned 

 to maximize the amount of information obtainable 

 from this ecological province. 



The methods used in collecting and processing 

 these data, with a summary of the previous year's 

 work are found in reports of the California Marine 

 Research Committee, the sponsoring organization 

 of the California Cooperative Oceanic Fisheries 

 Investigations, for 1950, 1052, 1953, 1955, and 

 1956. More detailed explanations are given by 

 Alilstrom (1948 and 1953), and in the following 

 discussion. 



The station pattern and numbering sj^stem are 

 described in Station Positions of the California Co- 

 operative Sardine Research Program, prepared bj' 

 the Scripps Institution of Oceanography and the 

 U.S. Fish and Wildhfe Service (1952). The sta- 

 tions laid out in lines occupied during the period 

 1951-54 are shown in figure 1. The exact location 

 of each station (at each occupancy) during 1951- 

 54 is given by Staff, South Pacific Fishery Investi- 

 gations (1952, 1953, 1954, and 1955). A plankton- 

 net tow (Ahlstrom, 1953), Nansen bottle cast, 

 and bathj'thermograph cast are made at each 

 station. Temperature data are obtained from 

 the reversing thermometers on Nansen bottles 

 and from bathy thermograms. The obliquely 

 hauled plankton net is retrieved from a depth of 

 approximately 140 meters (200 meters of wire out) 

 at an average rate of 20 meters of ware per minute. 

 The angle from the vertical of the towing wire is 

 kept as close as possible to 45 degrees. A current 

 meter placed in the mouth of the net measures the 

 volume of water strained. The sample obtained 

 is preserved in its entirety in a buffered formalin 

 solution, and these preserved samples are subse- 

 quently examined for fish eggs and larvae. The 

 numbers of jack mackerel larvae and localities in 

 which they were taken during 1952-54 are given 

 in Ahlstrom (1954a) and Ahlstrom and Kramer 

 (1955, 1956). The numbers of jack mackerel 

 eggs and localities in which they were taken in 

 1951-54 are given by Farris (1958). 



ESTIMATING EGG ABUNDANCE 



The method used to estimate egg abundance 

 has been described in detail by Sette and x\hlstrom 

 (1948) and Ahlstrom (1954b). The monthly esti- 

 mates of egg abundance are obtained from the 

 relation — 



n 



Cm=^ CtW,t, 



where 



C\/ = the monthly estimate of egg abun- 

 dance 



