Staudinger: Predation by four fish predators on two squid species on the Northwest Atlantic continental shelf 



609 



I Winter B Spring QFall 



Pre-recruits 



B Recruits 



Time of day 



Figure 4 



Relative catches of longfin inshore squid {Loligo pealeii) (A) prerecruits (s80 mm mantle length) and (B) recruits (>80 

 mm mantle length) predicted in demersal waters during winter, spring, and fall of 2002 and 2003 (summer sampling 

 was not conducted). Abundances of longfin inshore squid were measured as catch per tow (kg) and adjusted by seasonal 

 diel correction coefficients specified in Hatfield and Cadrin (2002). 



tion of the total mass consumed. To account for large 

 numbers of tied rank values, an adjusted H test statistic 

 was used to test the null hypothesis (Sokal and Rohlf, 

 1995). The two approaches produced similar results for 

 the majority of tests. Therefore, only conservative test 

 results are reported. 



Results 



Seasonal and diurnal differences 

 in longfin inshore squid abundance 



Predicted catches of longfin prerecruits during 2002 

 and 2003 indicated that the highest catch levels occured 

 during the fall, intermediate levels during the winter, 

 and minimum levels during the spring (Fig. 4A). These 

 calculated values are similar to the historical abun- 

 dance trends previously reported in Northwest Atlantic 

 waters. Fall catches of prerecruits over all times of 

 day were an order of magnitude higher in comparison 

 to winter and spring values. The greatest variation in 

 diurnal catches of prerecruits was predicted during 

 the fall season (Fig. 4A). Conversely, catches of longfin 

 recruits were estimated to be at their maximum during 

 the winter, at intermediate levels during the fall, and at 

 minimum levels during the spring (Fig. 4B). Maximum 

 catch rates of longfin recruits were predicted during 

 crepuscular and daytime periods of winter (Fig. 4B). 

 Moreover, nighttime catches of recruits were two to 

 three times higher during winter than during all other 

 seasons. 



Diet analysis 



Bluefish Bluefish are present in Northern Atlantic 

 waters only during the warmer months of the year; 



therefore sampling was restricted to the summer and fall 

 seasons. A total of 299 bluefish from 26 sampling dates 

 were analyzed. Fish ranged in size from 90 to 780 mm FL 

 (average=469.6 mm). Overall, 53% of all bluefish sampled 

 had food items present in their stomachs, and diets were 

 split nearly equally between squid and fish. Perciforms 

 were the dominant piscine prey; small amounts of amphi- 

 pods and other pelagic crustaceans made up the remain- 

 der of the diet. A complete list of prey species recovered 

 from all predator diets and their designated taxonomic 

 groups can be found in Staudinger (2004). 



Longfin squid was the primary squid species con- 

 sumed by bluefish. A substantial amount of shortfin 

 squid was also found in the stomachs of large fish 

 (Table 1). Total predation on squid was greater during 

 the summer than in the fall (Table 2); however, season- 

 al differences were not statistically significant possibly 

 because of the high frequency of zero values in the da- 

 taset (i^„rf,„sW=2-40, P=0.121, n=25). Medium and large 

 fish were the primary consumers of squid, and small 

 bluefish fed almost exclusively on fish (Table 1). Sig- 

 nificant differences were found in the amount of squid 

 consumed among all size classes of bluefish (H^^j^^f^j= 

 6.62, P=0.037, 7?=75). 



Goosefish A total of 536 goosefish stomachs were ana- 

 lyzed of which 269 (50%) contained prey. Goosefish ranged 

 in size from 75 to 818 mm TL (average = 341.1 mm). 

 Goosefish were almost exclusively piscivorous; 93% of 

 the total diet consisted of a mixture of gadiforms, clu- 

 peiforms, perciforms, and several other fish groups. 

 Goosefish were the only species in the present study to 

 prey on rajiforms. 



Occurrence of squid in the goosefish diet was re- 

 stricted to the winter and fall (Table 2) and differences 

 among all seasons were significant (//g^,^,,,,p^,=15.27, 

 P=0.002, n=A6). Although medium size goosefish were 



