Shad Catches and Water Temperatures in Virginia

Abstract

The relationship of shad (Alosa sapidissima) runs and water temperatures has long been recognized. Although general statements concerning this relationship are present in the literature, some are contradictory and few are supported by specific evidence. Leach (1925) stated that the main body of shad arrived at the river mouths when water temperatures were 56* to 661F., while Bigelow and Schroeder (1953) said that shad enter streams from the Gulf of Maine when the water has warmed to 50' or 550F. Talbot (1954) indicated that shad runs in the Hudson River usually start about the first of April, when the water temperature is about 400F., and the peak of the run occurs between the middle of April and the middle of May when average water temperatures are 450 to 570F. The role of temperature in controlling the movements of fishes other than shad has been described by Huntsman (1936), who found in the Margaree River, Nova Scotia, that a late salmon (Salmo salar) run corresponded with delayed warming of river water. Mottley (1938) working in Paul Lake, British* Columbia, concluded that temperature acted both directly and indirectly in influencing the spawning time of rainbow trout (Salmo gairdneri kamloops). Sullivan (1954) reviewed much of the work regarding the effects of temperature on the movements and distribution of fishes. To investigate the possible effect of water temperatures on shad catches, records of catches and temperatures for 1953, 1954, 1955, and 1956 have been examined from the York River, a tributary of Che apeake Bay located 25 miles northwest of the Virginia Capes. Water temperature records were obtained with a Foxboro thermograph located at the end of the Virginia Fisheries Laboratory pier at Gloucester Point 6 miles upriver from the mouth of the York River. The mean depth of the sensitive element of the thermograph was about four feet below the surface. Since the York River water is essentially isothermal during spring, these temperatures are considered to be representative for the region of the river traversed by shad. The daily temperatures were taken as the mean of the maximum and minimum points on the thermograph, and the weekly averages represent the mean of the seven daily values. Fishermen capture shad in the York River with stake gill nets. These nets, 20 by 15 feet respectively in average length and depth, are usually set in series of from 10 to 40. Records of the numbers of nets fished daily and the numbers of shad captured were kept each year from 1953 to 1956 by a group of 15 to 23 shad fishermen who set their nets within 20 miles upriver from Gloucester Point. Daily catches were averaged to obtain a mean catch-per-unit-ofeffort for each week. Comparable weeks during the 'Contribution from the Virginia Fisheries Laboratory No. 72.