HURLEY: LOLIGO OPALESCENS RAISED IN THE LABORATORY 



animals. Fisheries statistics are not particularly 

 helpful because the catches come mainly from a 

 few locations. It has been possible to get some 

 field information on the diet of the adult squid 

 (Fields 1965) but these data are completely 

 lacking on such necessary information as feed- 

 ing rates. 



It appears to be equally difficult to obtain in- 

 formation on young L. opalescens from field sam- 

 ples. The young squid have well-developed eyes 

 and are very sensitive to vibrations. Therefore, 

 even the young are likely to be able to avoid 

 many nets. Okutani and McGowan (1969) pub- 

 lished data on the abundance of young L. opales- 

 cens (size range 3.5 to 7 mm dorsal ML) taken 

 in net tows during the California Cooperative 

 Oceanic Fisheries Investigations cruises in 1954 

 to 1957. In their report, however, they emphasized 

 the problems involved in sampling the young 

 squid and stressed that the abundances given 

 probably should only be used to compare relative 

 abundances of different species. They found that 

 L. opalescens was the third most abundant 

 species of larval squid present in their samples, 

 but that its abundance was quite low when com- 

 pared to the most common fish larvae present 

 (e.g., 0.008 times the abundance of northern an- 

 chovy, ^n^raw/is mordax). 



If the role of a young L. opalescens as a predator 

 is to be evaluated, it is necessary to know the type 

 of prey which it eats. Fields (1965) has deter- 

 mined the diet of the adult squid from an exami- 

 nation of stomach contents, but to my knowledge 

 no one has done a similar study on the very small 

 squid. From the laboratory results presented in 

 this paper, it appears that young L. opalescens 

 must be considered as predators on a wide range 

 of prey types and prey sizes. They are capable of 

 preying on species ranging in size from 0.7 to 7 

 mm and they readily attack prey species ranging 

 from brine shrimp adults and nauplii to copepods 

 and larval fish. McGowan (pers. commun.) has 

 found that they also successfully attack the mysid 

 Metamysidopsis elongata. 



It is also possible to use the data presented here 

 to estimate a feeding rate for the young squid. 

 The respiration data can be used to calculate the 

 amount of food a young squid would need to sus- 

 tain itself. The respiration rate of the squid in the 

 rearing tanks can be taken as 3 ix\ 02/mg dry 

 wth. An average value for the caloric value of 

 oxygen consumed is 5 x 10"^ cal//il of O2. There- 



fore, a newly hatched squid (2.7 mm ML, weigh- 

 ing 0.625 mg) would use 0.22 cal for respiration 

 alone in 24 h. 



It is possible to determine how many prey items 

 of different types of prey would satisfy this re- 

 quirement. A newly hatched Ar^emia nauplius is 

 the equivalent of 0.0096 cal (John Hunter pers. 

 commun.). Therefore, a newly hatched squid 

 would need 23 Artemia nauplii per day. If the 

 squid were instead feeding on newly hatched 

 northern anchovies, it would need a total of 2 an- 

 chovy larvae per day (using a value of 5 cal/mg, 

 weight of larva = 0.022 mg; Theilacker and 

 Lasker 1974). Similar calculations can be made 

 for older squid. A squid 7 mm ML (~2 mo old, 6 

 mg) would consume 225 nauplii or 20 anchovy 

 larvae simply to meet its metabolic needs. The 

 actual amount of food consumed per day was ap- 

 preciably more than this, averaging about 50% of 

 body weight per day. At this rate, a newly 

 hatched squid would consume 150 nauplii or 14 

 anchovy larvae per day, while a 7-mm squid 

 would consume 1,500 nauplii or 135 anchovy lar- 

 vae per day. 



Data on feeding rates and abundance could be 

 used to calculate the impact that young squid 

 might have on populations of potential prey 

 items, but before such calculations can be mean- 

 ingful, more information must be known about 

 the ability of the squid to locate sources of food. 

 Loligo opalescens was only one hundredth as 

 abundant as the most common fish larvae (Oku- 

 tani and McGowan 1969). But with feeding rates 

 of 15 to 135 larvae per day, young squid could 

 potentially have a large impact on such popula- 

 tions if they concentrate on this type of food and if 

 they have effective means of finding such prey. 

 Laboratory observations indicate that larval fish 

 may be a preferred food, and the squid do occur in 

 areas where larval fish are common. Okutani and 

 McGowan found that L. opalescens was most 

 common in the upper 40 m, and this is the 

 stratum where the highest abundance of north- 

 ern anchovy larvae occur (Ahlstrom 1959). 



ACKNOWLEDGMENTS 



I thank J. Hunter, R. Lasker, and D. Lange for 

 help during this work. This study was done while 

 I was on a NOAA Associateship at the Southwest 

 Fisheries Center, National Marine Fisheries Ser- 

 vice, NOAA, La Jolla, Calif. 



181 



