FISHERY BULLETIN: VOL. 87, NO. 3, 1989 



new criteria that were stable for a longer period. 

 Here we describe and evaluate a diagnostic 

 index, the height of midgut mucosal cells of lar- 

 val northern anchovy. The index is sensitive to 

 feeding history, resistant to autolysis (with- 

 stands prolonged time in a collecting net), and 

 simple to measure. It is reliable for fish fixed in 

 formalin, the commonly used fixative for field 

 collections. 



METHODS 



Experimental Design 



We raised larval northern anchovy under dif- 

 ferent feeding conditions to produce specimens 

 that exhibited various health states. These 

 larvae were used to describe growth, determine 

 the dominant midgut height measurement 

 within each feeding treatment, evaluate the mid- 

 gut index, and estimate response times. In a 

 second series of experiments, gi'oups of fed and 

 starved fish were treated with nets in a manner 

 designed to simulate plankton collection methods 

 in the sea. These studies were used to evaluate 

 how useful the new criterion would be in a field 

 situation. 



Rearing Treatments 



Five groups of anchovy were raised at 15.5°C 

 for three weeks in 100 L containers. The eggs, 

 stocked at 10/L, were collected from a hormone- 

 injected broodstock maintained at the Southwest 

 Fisheries Center aquarium (Leong 1971). We fed 

 the control group ad hbitum on Gijmnodinium 

 and Brachionus (Lasker et al. 1970; Theilacker 

 and McMaster 1971) at yolk absorption (four 

 days after hatching at 15.5°C; hatching = day 0), 

 and delayed feeding 1-4 days in the other 

 gi'oups. Fish were sampled daily from the fed, 

 starved, and delay-fed treatments and pre- 

 served in Bouin's fixative (Table 1). 



Field Simulation Experiments 



One group of northern anchovy was fed for 3 

 days and one was starved for 3 days. We treated 

 samples from each group of 7 d old fish in a net 

 by flushing the submerged net with 15.5°C sea- 

 water for 0, 2, 5, 10, 15, 20, and 25 minutes (see 

 Theilacker 1980 for details of this method). After 

 the net treatment, fish were preserved in either 

 5% buffered formalin (standard shipboard fixa- 

 tive) or Bouin's solution (required fixative for 

 histopathology). 



Histological Preparation 



We measured the standard length (SL) of 

 preserved larvae to the nearest 0.01 mm and 

 subsequently prepared them for histopathology 

 using standard microtechniques (O'Connell 

 1976). Tissue was dehydrated, embedded in 

 paraffin, serially sectioned at 6 (xm in the sagittal 

 plane, mounted, and stained with hemato.xylin 

 and eosin. 



Measurement of Midgut Cell Height 



The midgoit is the major part of the intestine of 



Table 1. — Standard length (SL) and 



' SL taken on Bouin s lixed larvae 

 ^ Midgut cell height (see text) 



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