EFFECTS OF DELAYED INITIAL FEEDING ON LARVAE OF THE 

 GRUNION, Leuresthes tenuis (AYRES) 



Robert C. May' 



ABSTRACT 



The initial feeding of newly hatched larvae of the grunion, Leuresthes tenuis (Ayres), was delayed for 

 various periods of time under laboratory conditions at 18° C. Unfed larvae did not develop morpho- 

 logically beyond the stage reached at the time of yolk absorption, about 4 days after hatching, although 

 some survived as long as 3 weeks. Regardless of how long initial feeding was delayed, 80% or more of 

 previously unfed larvae began feeding when food was made available to them, and at least 40% of the 

 larvae alive when food was offered were able to survive to the end of a 20-day experiment. Some lar- 

 vae feeding for the first time after 1 to 2 weeks without food, died after gorging themselves with 

 Artemia nauplii. When food was offered to starved larvae, growth began and generally proceeded 

 at about the same rate as in larvae fed from day 1, although there was some indication that a few 

 days' delay in initial feeding increased the conversion efficiency of grunion larvae feeding on Artemia 

 nauplii. Catabolism of fat provided most of the energy for metabolic processes during starvation. The 

 condition factors and carbon/nitrogen ratios of unfed larvae were below those of fed larvae; condi- 

 tion factor seemed to be the better index of nutritional state. Grunion larvae probably do not exper- 

 ience high mortality at sea due to starvation, nor do they exhibit a classical "critical period" at the 

 time of yolk absorption. 



Most marine fishes pass through a free-swim- 

 ming larvae stage, and it is well documented that 

 survival through this stage is very low, gener- 

 ally being much less than 0.1 "^r (e.g., Sette, 

 1943; Ahlstrom, 1954; Pearcy, 1962; lizuka, 

 1966). The rate of survival through the larval 

 stage is probably the most important factor 

 determining the strength of year classes (Be- 

 verton, 1962; Gulland, 1965). Hjort (1914, 

 1926) advanced the hypothesis that larval sur- 

 vival was drastically affected by the abundance 

 of food at the time the yolk was completely 

 absorbed, and that poor year classes resulted 

 when insufficient food was available to larvae 

 at this "critical period." As Marr (1956) point- 

 ed out, Hjort's "critical period" concept has had 

 a profound effect upon the thinking of fishery 

 biologists. 



Increased larval mortality at the time of yolk 

 absorption has, however, proved difficult to dem- 

 onstrate in nature. Marr (1956) concluded that 



' Scripps Institution of Oceanography, University of 

 California, San Diego, La Jolla, Calif. 92037, and Na- 

 tional Marine Fisheries Service Fishery-Oceanography 

 Center, La Jolla, Calif. 92037. 



the published evidence did not establish the ex- 

 istence of such increased mortality at sea ; even 

 in the light of more recent field data (Farris, 

 1961; Pearcy, 1962; Stevenson, 1962; lizuka, 

 1966; Karlovac, 1967), it is difficult to decide 

 from survival curves whether increased mortal- 

 ity at yolk absorption does in fact occur in na- 

 ture. It has proved equally difficult to demon- 

 strate that starvation is a major cause of larval 

 mortality in the sea. Wild larvae found with 

 empty guts (Lebour, 1920; Bowers and Wil- 

 liamson, 1951; Arthur, 1956; Bhattacharyya, 

 1957; Berner, 1959) may indicate imminent 

 death by starvation or may reflect artifacts such 

 as defecation or selective capture by plankton 

 nets (Blaxter, 1965. 1969). Reports of appar- 

 ently emaciated larvae, sometimes caught in re- 

 gions where food is scarce (Soleim, 1942; Ar- 

 thur, 1956; Shelbourne, 1957; Nakai, 1962; 

 Hempel and Blaxter, 1963; Nakai et al, 1969), 

 are suggestive but inconclusive (Marr, 1956; 

 Blaxter, 1965, 1969). Field data thus indicate 

 the possibility of high larval mortality due to 

 starvation after the yolk has been absorbed but 

 have not demonstrated its existence conclusively 



Manuscript received January 1971. 



FISHERY BULLETIN: VOL. 69, NO. 2. 197L 



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