THEILACKER: MORTALITY OF SEA-CAUGHT JACK MACKEREL 



single specimen, lesions were present not only in the 

 brain but throughout the spinal cord (Fig. 7) as well. 

 In addition, the gut and associated glands had 

 deteriorated to the extent that this fish was con- 

 sidered starving. 



An abnormal central nervous system of a jack 

 mackerel larva consisted of vacuolar degeneration 

 and shrinkage of neurons. The degenerating neurons 

 exhibited increased staining (Fig. 8). 



Digestive Track and Associated Glands 



The midgut mucosa of young jack mackerel is com- 

 posed of a single layer of columnar epithelial cells. 

 Older fish (3.7-4.0 mm) showed increased mitotic ac- 

 tivity in the basal layer. Microvilli bordered the 

 midgut lumen only in fish that appeared healthy. 

 Mucosal cells were closely united in the fish con- 

 sidered to be normal (Figs. 9, 10). Basal separations 

 between these cells were common, not only in fish 

 that appeared to be starving but also in fish that 

 showed signs of feeding and digestion (Fig. 11). 

 O'Connell (1980) also reported that sea-caught north- 

 ern anchovy exhibited basal separations between 

 mucosal cells while the apical portions were well 

 joined. 



All wild jack mackerel categorized as recovering 

 had basal separations between midgut mucosal cells. 

 Laboratory fish that were artificially starved for 1-2 

 d before feeding showed these separations for several 

 days after feeding resumed. In the laboratory, lar- 

 vae did not grow while their tissues were regen- 

 erating (Theilacker 1981). 



Many sea-caught jack mackerel of all ages had 

 intracytoplasmic vacuoles in the midgut epithelium. 

 Basal and membrane lined, these vacuoles resem- 

 bled the vacuolar condition found in some recover- 

 ing, laboratory fish (Theilacker 1981). In addition, 

 many sea-caught larvae had smaller, luminal 

 vacuoles that were found in the laboratory fish (Fig. 

 12). These luminal vacuoles may indicate a degen- 

 erative condition. In higher vertebrates a metabolic 

 imbalance can cause vacuolar degeneration. Vacuola- 

 tion appears first as numerous small, clear vacuoles 

 dispersed throughout the cytoplasm. As the condi- 

 tion becomes more severe, these minute vacuoles 

 coalesce to form large (sometimes single) clear 

 spaces that displace the nucleus (Anderson 1971). 

 On the other hand, the numerous luminal vacuoles 

 can secrete mucous into the lumen or store fat. Use 

 of a routine mucicarmine staining was negative for 

 the presence of mucous cells. Unfortunately, the 

 presence of fat in the vacuoles could not be tested 

 because fat is removed during tissue preparation by 



clearing agents. Neither vacuolar condition was 

 graded. 



Another unusual condition of the midgut occurred 

 in many of the smaller wild jack mackerel. In these 

 fish, the margin of the lumen had lost its integrity, 

 microvilli were absent, and the sloughing of the 

 mucosal cells into the lumen (a condition common 

 in starved laboratory jack mackerel) appeared to 

 have progressed until the lumen contained masses 

 of undefinable, cellular material (Fig. 13). O'Connell 

 (1980) described a comparable condition which he 

 found in the midgut of a single, northern anchovy 

 specimen, the smallest examined. All jack mackerel 

 exhibiting this condition were smaller than the size 

 attained at first feeding, indicating shrinkage had 

 occurred. The hindgut also contained necrotic debris, 

 and other diagnostic tissues were in poor condition. 

 These jack mackerel were classified as dying. 



Hindgut mucosal cells of wild jack mackerel 

 typically showed eosin-staining inclusions that are 

 reported to be sites of intracellular digestion (Iwai 

 1968, 1969; Iwai and Tanaka 1968; Watanabe 1981). 

 Inclusions in the wild jack mackerel varied in inten- 

 sity; in healthy specimens the intensity appeared to 

 be related to time of day (feeding period), increasing 

 during daylight hours and decreasing during the 

 night. Although the presence and intensity of hind- 

 gut inclusions were noted, they were not graded. 

 Inclusions were not present in larval teleosts de- 

 prived of food in the laboratory (Theilacker 1978; 

 Umeda and Ochiai 1975; O'Connell 1976). However, 

 in many wild jack mackerel showing signs of starva- 

 tion the presence of pale inclusions indicated that 

 the fish had eaten at some time in the past. 



The key diagnostic characteristics of the pancreas 

 were obscure in ocean-caught jack mackerel because 

 of the intensity of staining. In laboratory fish, the 

 pancreas was very sensitive to food deprivation. For 

 example, a breakdown in the symmetry of the acinar 

 secretory unit was detectable after 1 d of food 

 deprivation (Theilacker 1978). In the wild fish, the 

 intensity of the staining of the pancreas was difficult 

 to control (see Fig. 12), and I was not able to obtain 

 consistent results, hence the condition of the pan- 

 creas was not evaluated. 



The jack mackerel liver was considered normal 

 when hepatocytes had clear, distinct nuclei (Fig. 9). 

 The appearance of the cytoplasm was quite variable; 

 in some larvae very few intracellular spaces existed 

 in the cytoplasm of the hepatocytes whereas in 

 others extensive intracellular spaces existed. 

 Presumably these spaces are areas where glycogen 

 and fat are stored within the cell. This presumed in- 

 corporation of stores was most marked in healthy 



