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Visually Estimating Fish Density at Artificial Structures in Lake Anna, Virginia
Author(s) -
Graham Robert J.
Publication year - 1992
Publication title -
north american journal of fisheries management
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.587
H-Index - 72
eISSN - 1548-8675
pISSN - 0275-5947
DOI - 10.1577/1548-8675(1992)012<0204:vefdaa>2.3.co;2
Subject(s) - lepomis macrochirus , sampling (signal processing) , fish <actinopterygii> , underwater , fishery , statistics , environmental science , biology , ecology , geography , mathematics , computer science , archaeology , filter (signal processing) , computer vision
Four artificial structures in Lake Anna, Virginia, were chosen to develop and refine a sampling technique designed to assess the effectiveness of structures in attracting fish. Several underwater observation techniques were tried before one suited to the prevailing restrictive and low‐visibility conditions was devised and applied to the four structures in summer and autumn 1988. Three consecutive fish counts were made at nine sampling locations per structure. Bluegills Lepomis macrochirus less than 15 cm in total length dominated counts at all structures and were used to evaluate the sampling technique, Counts of bluegills made immediately after divers descended were significantly ( P < 0.05) lower than counts made 3 and 5 min after descent. Differences among initial and subsequent counts appeared related to bluegill adaptation to divers and diver adaptation to low light levels underwater. The cumulative mean number of bluegills counted over nine sampling locations per structure and the associated variability tended to stabilize by the time fish at the sixth or seventh location had been counted. Sampling seven locations per structure was sufficient to obtain an index of bluegill density. Differences in bluegill density from summer to autumn were apparent at two structures that were more than 4 years old but not at two 6‐month‐old structures.