FISHERY BULLETIN: VOL. 84, NO. 1 



Adjustment for Shrinkage 



® In order to use morphological measurements to 

 diagnose starvation of jack mackerel, it is essential 

 to adjust for shrinkage of body measurements. Both 

 handling and preservation cause shrinkage of lar- 

 val fishes, and the amount of shrinkage varies among 

 body parts. Final fish size is dependent not only on 

 initial size but also on the handling time (which is 

 different for the laboratory and the field) and the 

 type of preservative used (Blaxter 1971; Theilacker 

 1980a; Hay 1981). The shrinkage of laboratory speci- 

 mens of jack mackerel preserved in Bouin's solution 

 is known (Theilacker 1980a), but for field-collected 

 specimens the shrinkage caused by the net tow and 

 the subsequent effect of Bouin's preservative must 

 be evaluated. 



I conducted laboratory experiments to estimate 

 the amount of shrinkage caused by handling (net 

 retention) and by preservation. Live jack mackerel 

 were pipetted individually (time = 0) onto a slide, 

 and four body measurements were taken before 

 placing the fish into a net container through which 

 15°C seawater circulated. Standard length, head 

 length, eye diameter, and body depth at the anus 

 were measured. Body depth at the pectoral fin was 

 not measured because it was difficult to measure 

 quickly on live jack mackerel. During net treatments, 

 I usually remeasured each fish four more times at 

 5-7 min intervals, replacing the fish in the net be- 

 tween each set of measurements. After 25-30 min, 

 the fish were preserved in either Bouin's fixative 

 (used for histological analyses) or 5% buffered 

 Formalin 2 (as per shipboard procedures; Smith and 

 Richardson 1977). Remeasurements after preserva- 

 tion were taken in 3-4 wk. 

 (a\ Shrinkage of net-captured larval fish has been 

 shown to decrease with increasing fish size For ex- 

 ample, shrinkage of northern anchovy decreased 

 from 19% at 4 mm SL to 8% at 18 mm SL 

 (Theilacker 1980a). The jack mackerel tested in this 

 study ranged between 3.35 and 4.10 mm SL, and 

 within this restricted length group shrinkage was 

 proportional to size Thus for the shrinkage analy- 

 ses, all jack mackerel were combined into one 

 group. 



For the combined size group, length of the jack 

 mackerel body (Fig. 2) and the head continued to 

 shrink for the duration of the net treatment. Width 

 of the body (Fig. 3) and the eye shrank initially, and 

 then remained relatively constant during additional 



treatment. To account for positive correlation be- 

 tween body parts, a multivariate analysis (Table 2) 

 was used to relate the ratio of net-treated size to live 

 size (for each body part) with treatment time In- 

 dividual shrinkage was highly variable; for example, 

 shrinkage of body depth varied between and 23% 

 for treatment times between 5 and 20 min (Fig. 3). 

 However, since these were the best estimates of 

 average shrinkage for body parts, the regressions 

 (Table 2) were used to calculate the adjustment fac- 

 tors needed for this study. Factors for each body part 



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0.0 



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Figure 2— Shrinkage of standard length, shown as the ratio of net- 

 treated size to live size, of individual Trachurus symmetricus lar- 

 vae as a function of net-treatment time; estimated parameters are 

 in Table 4. 



0.0 6.0 12.0 18.0 24.0 



TIME (MIN) 



30.0 



2 Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Figure 3— Shrinkage of body depth, shown as the ratio of net- 

 treated size to live size, of individual Trachurus symmetricus lar- 

 vae as a function of net-treatment time; estimated parameters are 

 in Thble 4. 



