GRISWOLD and SMITH: LIFE HISTORY OF NINESPINE STICKLEBACK 



second mode. However, the second mode dis- 

 appeared in August as young-of-the-year be- 

 gan to appear in the catches. Since the 2-yr- 

 old fish, which remained after August were pre- 

 dominantly female (Table 16), it appeared the 

 die-off among old fish, as indicated by length- 

 frequency diagrams, was more prevalent 

 among males. The monthly sex ratios of the 

 age-groups comprising most of the spawn- 

 ing population seemed to indicate that such 

 mortality began to occur as early as June with 

 a sharp drop in males in the catchable popu- 

 lation through July. However, the die-off did 

 not appear in the length-frequency diagrams 

 until August. This discrepancy might be ex- 

 plained by a differential movement of males 

 onto the spawning grounds in June to set up 

 their territories and/or an increased aware- 

 ness of the fishing gear by territorial males. 

 Both these explanations could result in de- 

 creased male availability to the trawl during 

 the spawning period. However, after the spawn- 

 ing period, males remained low in the catch 

 throughout the rest of the year, indicating a 

 high mortality among males which spawned. 

 These interpretations are strengthened by 

 two other observations. Males which had 

 spawned previously were identifiable at any 

 time by vestiges of spawning colors on their 

 ventral surface. Few of these individuals were 

 seen in the collections. Also, in a laboratory 

 experiment described by Griswold and Smith 

 (1972), eight males brought off broods to the 

 point where fry were actively swimming about 

 in the aquaria. Seven of these males died within 

 a month of spawning. There was no mortality 

 observed among 10 females which spawned in 

 the laboratory. 



Table 16. — Ratio of male sticklebacks to females of 

 different age-groups in various months. Annulus formation 

 is complete after July. 



Weighted for number of fish in each month. 

 2 Weighted for number of fish in each age-group. 



PARASITE INFECTION 



The pleurocercoid stage of the cestode, Schi- 

 st ocephalus pungitii Dubinina (Pavlovskii, 

 1964), is the only parasite generally abundant 

 among ninespine sticklebacks. The procercoid 

 infects numerous species of copepods, and when 

 these are eaten by sticklebacks, the procercoid 

 burrows through the stomach wall to grow as a 

 pleurocercoid in the body cavity. At this stage 

 the parasite is species-specific as, so far as is 

 known, it attacks only the ninespine stickle- 

 back. Two months are required for full growth 

 of the pleurocercoid, at which time it may in- 

 fect a number of fish-eating birds. No other 

 stickleback parasites were noted in the study. 



Rate of infection in Apostle Islands stickle- 

 backs was low in 621 fish examined. Only 4 of 

 362 fish (1.10% ) were infected prior to 1 August 

 in all years of the study combined, and 13 of 

 259 fish (5.01%) were infected after 1 August. 

 Summer and fall infection would intuitively be 

 higher because of greater copepod abundance at 

 that time. The largest pleurocercoid found was 

 22 mm long and weighed 0.225 g or 14% of the 

 host's weight. Two fish had two pleurocercoids 

 each which made up 18 and 14.5% of the fish's 

 weight, respectively. These parasites were found 

 in 7 male and 10 female sticklebacks. Fish in- 

 fected at spawning time did not appear to be 

 maturing. 



FOOD HABITS OF THE STICKLEBACK 

 AND ASSOCIATED SPECIES 



Five papers mention the food of P. pioigitius. 

 Forbes (1883) found two specimens which con- 

 tained equal amounts of diptera lai'vae and 

 cladocerans. Blegvad (1917) found copepods 

 and stickleback eggs predominant in 112 Danish 

 fish. Hynes (1950) reported cladocera, amphi- 

 pods, and chironomid larvae were the most 

 important food items in fish from an English 

 stream. Bigelow and Schroeder (1953) state that 

 the stickleback feeds voraciously on fish eggs 

 and fry and can be very destructive to other 

 species in fresh water. Anderson and Smith 

 (1971) report crustaceans make up more than 

 90% of the Lake Superior stickleback diet. In the 

 present study, emphasis was placed on the food 



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