STARVATION-INDUCED MORTALITY OF YOUNG 



SEA-CAUGHT JACK MACKEREL, TRACHURUS SYMMETRICUS, 



DETERMINED WITH HISTOLOGICAL AND MORPHOLOGICAL METHODS 



Gail H. Theilacker 1 



ABSTRACT 



Young jack mackerel, Trachurxis symmetricus, living offshore are starving while those living nearshore 

 are healthy. These results for sea-caught jack mackerel were determined by using histological and mor- 

 phological criteria that reliably diagnosed the viability of laboratory-raised jack mackerel. Both the 

 histological and morphological indices indicated that 350 km offshore about 70% of the first-feeding jack i 

 mackerel were starving. In contrast, 12% of the fish collected near islands and banks were starving. In 

 both habitats, mortality rates decreased to zero for jack mackerel at 2 weeks of age The accuracy of 

 the techniques for prediction of the nutritional state of wild larvae is discussed and evaluated. 



Jack mackerel, Trachurus symmetricus, hatch with 

 yolk reserves that last for 5dat 15°-15.5°C. After 

 the yolk is absorbed, they must eat within 3 d or die 

 of starvation. In addition, growth is retarded in lar- 

 vae that have experienced only 1 d of starvation, and 

 resumption of normal growth does not occur until 

 2-3 d after the starvation period (Theilacker 1978, 

 1981). Thus, in the laboratory, availability of food at 

 the time of first feeding affects growth and survival 

 of young jack mackerel. In the field, the relative im- 

 portance of starvation as a source of mortality of 

 jack mackerel is unknown. It was first suggested by 

 Hjort (1914) (reviewed by May 1974) that the 

 strength of the year class is determined early in life 

 by the availability of food for larvae at the time of 

 first feeding (the critical period hypothesis). But only 

 recently (O'Connell 1980) has the presence of 

 starving ocean-caught larvae been documented. In 

 this study I give evidence that starvation may be a 

 major cause of natural mortality of young jack 

 mackerel at sea. I use two techniques, developed in 

 the laboratory, to determine the incidence of starva- 

 tion (Theilacker 1978). The potential use of these 

 techniques to monitor sea samples for larval survival 

 is discussed. 



METHODS 



Collection 



In May 1980 a concentration of jack mackerel eggs 

 and larvae was located 350 km off the coast of 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 92038. 



Manuscript accepted February 1985. 

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California (lat. 31°00'N and long. 120°30'W). A 400 

 mi 2 grid was established which contained 41 sta- 

 tions, 4 mi apart; it took 4 d to sample all stations 

 (Fig. 1). At each station, a standard oblique bongo 

 net tow (Smith and Richardson 1977) and aim net 

 sample were taken. The bongo samples will be used 

 in another study to estimate growth and mortality 

 of jack mackerel larvae (Hewitt et al. in press). The 

 1 m net (505 /urn mesh) was used to sample larvae 

 qualitatively from the upper 50 m of water. Ahlstrom 

 (1959) found that 88% of the larval jack mackerel 

 collected off California were in the upper 50 m, and 

 all the jack mackerel collected by Devonald (1983) 

 were above 42 m. A special collection procedure was 

 used for the samples taken for histological and mor- 

 phological analyses. Immediately after the net tow, 

 the sample was preserved in Bouin's solution to avoid 

 autolysis of larval tissues (elapsed time was usually 

 8 min) (Theilacker 1978). The collecting net was not 

 washed down (a procedure required for quantitative 

 samples), and the cod end containing the sample was 

 placed directly into Bouin's solution. The preserved 

 sample was removed from the cod end within an 

 hour. After 2 d, Bouin's solution was replaced by 70% 

 alcohol. 



In addition to jack mackerel collections taken in 

 the open ocean 350 km offshore, a few special tows 

 (n = 24) for assessment of starvation were made dur- 

 ing routine cruises in 1978, 1979, and 1980 near the 

 Channel Islands (Anacapa, Santa Barbara, and San 

 Clemente) and Tanner Bank. 



Preparation of Fish 



More than 2,000 jack mackerel were collected in 



1 



