In the past years, the numbers of gizzard shad 

 in Lake Erie were too few for the species to create 

 any important problems, indeed, to have any real 

 significance in the bionomics of the lake, but 

 recently their abundance has increased enor- 

 mously. Mass mortalities have created esthetic 

 and public-health problems, water intakes have 

 been plugged, and commercial fishermen have 

 wasted hours sorting and discarding the worth- 

 less shad from their catches. From the fish- 

 management standpoint, the question arises 

 whether the value of shad as forage may not now 

 be outweighed by their diversion of the productive 

 capacity of the lake into commercially valueless 

 fish. An inquiry into the natural history of the 

 gizzard shad in Lake Erie, accordingly, has been 

 much needed. 



Only a few precocious male and female gizzard 

 shad attain sexual maturity as age group I. 

 Almost all males and a good percentage of females 

 mature at age II and only rarely are Ill-group 

 shad immature. Development of the egg and 

 seasonal changes of the ovary are described. 

 Egg production is highest in the II group — 

 average of 378,990 per individual and 689 per 

 gram of body weight. Spawning takes place from 

 early June into July and is most intensive near 

 mid-June. Heaviest spawning is at water tem- 

 peratures of 67° F. or more. Early development, 

 to the attainment of the adult shape is described; 

 particular attention is given to the development 

 of the alimentary tract. 



The anatomy of the digestive tract in the adult 

 is described and comments are offered on the 

 function of such organs as the pharyngeal pouches 

 and the caeca. Tests were made for digestive 

 enzymes in different parts of the tract. The 

 gizzard shad is a filter feeder. Food varies 

 widely with season and locality but consists 

 mostly of phytoplankton and zooplankton. 



The gizzard shad population has been increasing 

 in Lake Erie. Commercial fishermen told me that 

 3 decades ago the "sawbelly" was something of a 

 rarity — only an occasional one was caught. They 

 have attained greatest abundance, according to 

 these fishermen, since about 1950. Whether this 

 increase results from adaptations to the Lake Erie 

 environment, to changes in the character of the 

 lake, or merely to increasing surpluses of shad 

 beyond environmental and predatory inroads has 

 not hecn determined. 



Gizzard shad seem to be most plentiful in the 

 shallow waters around the periphery of western 

 Lake Erie, in the Bass Islands area, and especially 

 in protected bays and mouths of tributaries. 

 The numbers present here vary from season to 

 season. They are most numerous in late summer 

 and early fall when their abundance is increased 

 by the recruitment of the young of the year. 

 They are next most plentiful just prior to and 

 during their spawning season in late spring when 

 the mature shad congregate in the shallow waters. 

 Then, too, in winter they concentrate in places 

 into which warm streams flow. At other times 

 they are rarely seen in numbers — frequently days 

 go by without any captures of shad by the com- 

 mercial fishermen of South Bass Island. 



The tendency of shad to inhabit shallow water, 

 their attraction in massive numbers to the warmer 

 water of outlets from industrial plants and of 

 inflowing streams, and the frequent mass mortal- 

 ities doubtless have given rise to an exaggerated 

 idea of their abundance. Nevertheless, they are 

 plentiful and their numbers are growing. The 

 shad problem is real in both a practical and a 

 purely scientific sense. 



MATERIALS AND METHODS 



The investigation of the fife history of the 

 gizzard shad was based on almost 24,000 fish 

 (about two-thirds of which were young of the 

 year) captured in western Lake Erie and in streams 

 tributary to the southwestern shore of this lake 

 in 1952-55. Studies of age, length, weig'ht, 

 growth, reproduction, and fecundity were made 

 for shad captured in the lake. Fish caught in 

 Sandusky Bay and in the tributary streams were 

 omitted from those phases of the study because 

 of differences in growth and other aspects of their 

 life history. 



The Sandusky Bay specimens were decidedly 

 smaller than lake fish of the same age and appeared 

 to have formed their annuli earlier in the year. 

 Also, they probably spawned earlier than the lake- 

 dwelling gizzard shad. The water in Sandusky 

 Bay was frequently turbid — the shallow water is 

 readily turned over by winds. Phytoplankton, 

 though reduced by this turbidity, is not lacking, 

 for much is washed into the bay from the marshes 

 by the tributary streams. Stomach contents of 

 fish caught in Sandusky Bay were largely muddy. 

 The slower growth of the fish here probably 



.'I'.iJ 



U.S. FISH AND WILDLIFE SERVICE 



