ONTOGENY AND SYSTEMATICS OF FISHES-AHLSTROM SYMPOSIUM 



mortalities occurred at first feeding. Measurements of mortality 

 rates of eggs and larvae at sea tend to show a high but continuing 

 mortality of perhaps 5-20% per day. The results of sea surveys 

 are, however, often difficult to interpret because of the need to 

 sample within a discrete larval population over a long time. 

 May (1974), in his review of this subject, concluded that star- 

 vation at the end of the yolk-sac stage may often have a major 

 influence on brood strength but that mortality from fertilization 

 to the O-group stage is the ultimate determinant. 



The results of modelling and the tests of the patchiness hy- 

 pothesis which have already been discussed support the idea 

 that first feeding is a critical time, although not having, neces- 

 sarily, the dominant effect claimed by Hjort. Experimenters and 

 modellers have also derived further concepts for testing. The 

 major sources of mortality are identified as starvation and pre- 

 dation. Starvation, of course, only operates from the end of the 

 yolk-sac stage. Blaxter and Hempel (1963) used the expression 

 "point-of-no-retum" to express the point at which larvae, as a 

 result of starvation, are too weak to feed even if food becomes 

 available. Sometimes called "ecological death" or "irreversible 

 starvation" this is a useful concept for assessing the chances of 

 larval survival under different conditions. For larvae in a good 

 nutritional state the time to the point-of-no-retum may be only 

 1-2 days in a small newly feeding larva like the anchovy, but 

 2-3 weeks in a well grown flatfish larva like the plaice (see 

 Theilacker and Dorsey, 1980). Implicit, also, in the concept is 

 that larvae can live for some time after the point-of-no-retum. 

 During this time they may be especially liable to capture by nets 

 and, without adequate knowledge, a false impression might be 

 obtained of the size or nutritional state of the larval population. 



The assessment of nutritional state of larvae has been of wide 

 interest in recent years, in the hope of relating this to brood 

 strength. Initially Blaxter ( 1 965) measured the condition factors 

 of tank-reared herring larvae after varying periods of starvation 

 and then later compared the results with the condition factors 

 of sea-caught herring larvae (Blaxter, 1971). It was found that 

 most sea-caught larvae had much lower condition factors than 

 starving tank-reared larvae and it became apparent that the 

 extrapolation of tank criteria to the sea was invalid because the 

 tank larvae were short and fat compared with wild larvae (see 

 Fig. 1). This means that condition factor comparisons of wild 

 larvae are only valid on a relative basis from year-to-year or 

 place-to-place (e.g., Chenoweth, 1970; Vilela and Zijlstra, 1971) 

 and only then if one can be satisfied that shrinkage after capture 

 is consistent. The problems of tank ; sea comparisons and 

 shrinkage are unfortunately likely to be the most serious in long 

 clupeoid larvae to which these experiments have been applied. 

 No one has checked their validity in the more common type of 

 larvae with a shorter body form. 



These problems led to work at Oban and La Jolla on histo- 

 logical criteria for assessing starvation (Ehrlich et al., 1976; 

 O'Connell, 1976; Theilacker, 1978). O'Connelfs work on an- 

 chovy larvae deserves special mention. He found from screening 

 the state of the body organs such as pancreas and gut that these 

 showed increasing signs of degeneration as starvation pro- 

 ceeded. On applying his criteria to sea-caught anchovy larvae 

 O'Connell (1981b) found evidence for quite a high percentage 

 of larvae suffering from advanced starvation and considerable 

 differences in the incidence of starvation in closely adjacent 

 areas. This method is now being applied by Theilacker on jack 

 mackerel larvae from year-to-year and is likely to be adopted 

 on a routine basis. 



The other cause of mortality, predation, has recently become 

 fashionable following the work of Eraser, Lasker, Lillelund and 

 Theilacker and subsequently Kuhlmann, von Westemhagen and 

 Rosenthal, Bailey, Purcell and several other workers (See re- 

 views of Hunter, 1981, 1984). Copepods, euphausiids, amphi- 

 pods and chaetognaths are all implicated but perhaps medusae 

 are the most voracious group of predators (Bailey and Batty, 

 1983), especially for inshore spawners like Pacific herring. Pre- 

 dation, of course, operates from the moment of spawning and 

 Hunter and Kimbrell( 1980) and MacCall (1980), in particular, 

 have discussed the incidence of density-dependent cannibalism 

 of spawning anchovies on their own eggs and larvae. It is gen- 

 erally thought that strong selection pressure exists for fast growth 

 which will take larvae speedily through the more vulnerable 

 early stages. Larvae have been shown experimentally to be less 

 vulnerable when they are larger, their escape speeds are higher 

 and their recovery from a predator attack (for predators of a 

 given size) more likely. As Hickey (1979, 1982) has shown, an 

 efficient wound-healing mechanism exists, allowing larvae to 

 recover from bites, stings and other forms of damage. The high 

 survival rates of larvae reared in the absence of predators (Kven- 

 seth and Oiestad, 1984; Morita, 1984) suggest strongly that 

 predation is a major source of mortality in the sea. Although it 

 is difficult to assess the relative importance of starvation and 

 mortality in any larval population, it is also clear that the two 

 must interact in the sense that starving larvae will be more 

 susceptible to predation. 



The Future 



In this paper modelling has been only briefly discussed. The 

 method is now widely used for setting up hypotheses about 

 feeding, starvation, predation, cannibalism and other factors 

 associated with the stock-recruitment relationship and biomass 

 estimation. This approach is likely to continue as a basis for 

 sea surveys. It seems uncertain whether biomass will be routinely 

 estimated by egg and larval surveys except perhaps in Pacific 

 herring and northern anchovy. The cost is too high and sonar 

 surveys, if the problems can be ironed out, seem to be a better 

 bet. 



Experimental data on predation still need to be collected and 

 few correlations exist between predator populations and egg and 

 larval mortality in the sea. In fact mortality studies on eggs and 

 larvae in the sea in general need to be perfected since the prob- 

 lems of following discrete populations and of ageing larvae are 

 still not fully solved. At least one source of information is largely 

 untapped and that is the explanation for the high survival rates 

 of larvae in large enclosures. In particular the distribution of 

 the larvae and their food in these enclosures is not known and 

 may throw light on the validity of the patchiness hypothesis. 

 Information on frontal systems, and interfaces as a result of tide, 

 wind, upwelling and thermo— and halo— clines is now quickly 

 being assembled by hydrographers and marine biologists. The 

 larval biologists should be ready to exploit the results. 



It will be apparent to the audience how far research into the 

 early life history of fish has advanced in the last 30 years. A 

 major force has been the work off"the Califomian coast generated 

 by Ahlstrom and his recruits at La Jolla. It is therefore very 

 fitting that this symposium should be dedicated to his memory. 



Scottish Marine Biological Association, Dunstaffnage 

 Marine Research Laboratory, P.O. Box 3, Oban, 

 Argyll, Scotland. 



