THEILACKER: MORTALITY OF SEA-CAUGHT JACK MACKEREL 



phological criteria was essentially the same as that 

 based on histological criteria. Owing to the relative 

 ease, and low cost of measuring fish compared with 

 a histological examination, the morphological 

 analysis is an attractive approach. On the other hand, 

 histological analysis defines a cause and effect rela- 

 tion between structure and starvation whereas gross 

 morphological measurements provide an index of 

 starvation which is highly vulnerable to errors and 

 biases in calibration and interpretations. Because of 

 the importance of these measurements in recruit- 

 ment studies, it is appropriate to consider the merits 

 of and potential errors in these techniques in some 

 detail. 

 (2/ The morphometric approach relies on measure- 

 ments of fish to compare reared and wild animals 

 at the same developmental stage Thus shrinkage ad- 

 justments are needed to intercalibrate laboratory 

 measurements and field measurements. Fish shrink 

 when collected in a net and preserved, and shrinkage 

 of the size of all body parts is dependent on the time 

 in the net, size of fish, and type of preservative used 

 (Blaxter 1971; Theilacker 1980a; Hay 1981). In this 

 study, tow time was controlled at 5 min and samples 

 were preserved within 8 min. Thus damage to the 

 fish and shrinkage were minimal, but the samples 

 were not quantitative It is doubtful that the morpho- 

 metric technique will work with jack mackerel taken 

 in standard, quantitative collections. Quantitative net 

 tows are 20 min, and they include an additional 

 hosing down of the nets before sample preservation 

 (Smith and Richardson 1977). The procedure 

 damages the larvae, causing extensive shrinkage 

 which makes accurate measuring difficult. Further, 

 a long tow time decreases confidence in time-specific 

 shrinkage estimates because fish can be collected at 

 any time during the towing period. Increasing the 

 tow time also causes both the magnitude of the 

 shrinkage correction factor and the standard error 

 of its estimate to increase For example, in this study, 

 standard length of jack mackerel shrank by an 

 average of 6.0 ± 0.6% in 8 min and 19.0 ± 1.0% in 

 20 min. 



While laboratory calibration is absolutely essen- 

 tial for the morphometric analysis, no shrinkage 

 calibration is needed for the histological analysis, and 

 it might be possible to use the histological observa- 

 tions on other fishes. Diagnostic criteria for the 

 starving condition of jack mackerel (Theilacker 

 1978), northern anchovy (O'Connell 1976), and 

 yellowtail, Seriola quinqueradiata, (Umida and 

 Ochiai 1975) were similar. In addition, important 

 biological information is gained while using the 

 histological approach whereas gross morphological 



indices provide no such information. For example, 

 histological analysis of jack mackerel has revealed 

 a pattern of diel swim bladder inflation and a disrup- 

 tion of this rhythm, accumulation of glycogen 

 reserves, and brain lesions presumably produced by 

 UV radiation (Hunter et al. 1979). There is just no 

 substitute for this extensive biological information. 

 On the other hand, population work requires large 

 samples, and morphological indices are probably the 

 only practical means for working with very large 

 samples. Thus, the optimal experimental design for 

 population work on starvation is probably the use 

 of morphological criteria (calibrated for shrinkage) 

 combined with a smaller subsample of fish which are 

 graded histologically. All work requires special net 

 tows, preservation, procedures, and laboratory 

 calibration. 



Caution needs to be exercised when transferring 

 information obtained in the laboratory to the field. 

 Raising larval jack mackerel in small containers is 

 known to affect growth, nutritive condition, and 

 possibly activity (Theilacker 1980b). Additionally, 

 there is evidence that wild fish tend to be thinner 

 than their laboratory counterparts (larval herring, 

 Blaxter 1971; juvenile herring, Balbontin et al. 1973; 

 larval northern anchovy, Arthur 1976). My use of the 

 morphometric SWDA assumes that the morpho- 

 metric criteria I developed in the laboratory for lar- 

 val jack mackerel raised in large tanks are applicable 

 to ocean-caught jack mackerel. 



ACKNOWLEDGMENTS 



Many thanks to Brian Rothschild who suggested 

 research on the nutritive condition of larval fish and 

 to William T (Tosh) Yasutake who offered me a per- 

 sonalized course in teleost histology. The offshore 

 collections were made possible by Roger Hewitt's ef- 

 fective planning, the crew of the RV David Starr 

 Jordan, and the assistance of Jack Metoyer and 

 Carol Kimbrell. Miguel Carrillo sorted the mackerel, 

 Richard Kiy measured them, and Jack Metoyer 

 prepared them for histological analyses. Metoyer also 

 helped with the shrinkage study. Nancy Lo assisted 

 with all statistical applications. I appreciate John 

 Hunter's and Martin Newman's constructive reviews 

 of the manuscript. Many thanks to the Technical Sup- 

 port Group for typing services. 



LITERATURE CITED 



Ahlstrom, E. H. 



1959. Vertical distribution of pelagic fish eggs and larvae off 

 California and Baja California. U.S. Wildl. Serv., Fish. 



15 



