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The JAI is a seasonal index value that results from the arithmetic integration of the geometric mean number of fish caught per cruise during juvenile shad monitoring on nursery grounds in York River. Approximately ten cruises occur within a season and twelve to fifteen stations are sampled on each cruise. The number of juvenile shad collected at each station is standardized for volume of flow through the net according to the following equation:
where Si = the number of fish caught per ith tow standardized for flow, Ni = number of fish caught on ith tow, W = standard volume of water filtered by the net when traveling a specific constant speed for a given amount of time ( 655 m3 at 1200 rpm for 5 min for this study), F = standard unit of conversion on the flowmeter (0.0267 m/revolution), Ri = revolutions of the flowmeter on the ith tow, and A = area of the net ( 2.25 m2 for this study). The average number of fish represented by each tow during a cruise was estimated by the geometric mean of Si:
where Ci = geometric mean number of fish caught in a tow during the sampling represented by cruise i and k = the number of stations sampled during cruise i. The total number of fish represented by each season was estimated by:
where JAI = the sum over the entire season of mean cruise cpue’s of juvenile shad, Di = the number of days between cruise i and cruise i+1, and Cl = the lesser value between Ci and C i+1. |
