FISHERY BULLETIN: VOL. 84, NO. 1 



after the size of their body parts was adjusted for 

 shrinkage. 



RESULTS 



Habitat Conditions 



A larval-density gradient was apparent in the open 

 ocean study area. High densities of jack mackerel 

 larvae (100-<300/sample) were found in the central 

 stations and in stations near the western boundary 

 of the grid; lower densities (20-50) were found to the 

 north and east, and densities of larvae approached 

 zero at the southern stations that were occupied at 

 the beginning and again at the end of the 4-d obser- 

 vation period (Fig. 4). Larval densities in the south 

 did not change during this period. 



The study area was chosen because temperature, 

 viewed on satellite thermal image of the sea surface, 

 corresponded to the temperature range (15°-16°C) 

 associated with jack mackerel spawning (Farris 

 1961). Surface temperature in the study area in- 

 creased from 15.2°C in the north to 16.8°C at the 

 southern stations, with the majority of jack mackerel 

 found in water temperatures of 16.1°-16.6°C. 

 Water temperatures inshore of the grid were about 

 14°C. 



A temperature-salinity curve obtained at station 

 19 (Fig. 1) agreed well with the curves obtained from 

 inshore stations with the exception of the warm- 

 water portion of the curve, which appeared to be a 

 thin, warm lens of open ocean water intruding coast- 

 ward over deeper coastal water. 



Histological Assessment of 

 Fish Condition 



I used the tissue characteristics of laboratory fish 

 (raised at 15.0° -15.5° C) of known feeding history as 

 the criteria to determine the nutritional condition 

 of the sea-caught jack mackerel. Photomicrographs 

 of the diagnostic tissue characteristics were 

 documented by Theilacker (1978). Many of these 

 characteristics are shown also for wild fish (Fig. 5, 

 see also Figures 6-14). In addition, the wild fish ex- 

 hibited four tissue conditions that were not observed 

 in the laboratory: lesions in the brain; luminal 

 vacuoles in the midgut; total degeneration of the 

 midgut mucosal cells; and a wavy configuration of 

 the muscle fibers. Each of these conditions will be 

 considered in the following section that describes the 

 tissues of ocean-caught fish. My emphasis will be on 

 those tissue characteristics that diagnose starvation 

 in young jack mackerel. 



Brain 



The brain of an ocean-caught jack mackerel was 

 considered normal when the neurons were distinct, 

 round, and closely spaced. In these fish, brain cell 

 division was common, but it was not graded. One 

 percent of the jack mackerel examined had brain le- 

 sions of the type (Fig. 6) induced by ultraviolet light 

 in larval northern anchovy, Engraulis mordax, and 

 Pacific mackerel, Scomber japonicus (Hunter et al. 

 1979). The grading system classified these jack 

 mackerel (n = 3) into the healthy category. In a 



^Pt. Conception 

 'O^s? ~"* a *fcl_os Angeles 



'San Diego 



31» N • — 



120.5* W 



NUMBER OF LARVAE 



0-5 



20-50 



100-<300 



Figure \.—Trachurus symmetricus larval density gradient shown 

 as number of larvae collected per sample (not quantitative). Sta- 

 tion grid located 350 km off the coast of California. 



