BLAXTER: ONTOGENY, SYSTEMATICS, FISHERIES 



Lateral line 



Respiration 



Red muscle 



Reynolds number 

 (Re) and 



hydrodynamic 



Viscous 



regimes 

 Digestive tract 



Re<lO 



Time to 50% 

 starvation 



Larval period 



First feeding 



"^ 1 



Photopic vtslon I 



Lens retractor muscle 

 Improved accommodation^ — " 



^ *-'- 



Threshold 



tof 

 schooling 



Initial swim bladder Inflation 

 Olel vertical movements 



, ---^T- 



Functional eye First rods 



Increase in number of neuromasts 



Scotopic vision 



Rod recruitment continues 



Many rods 



Canal formation 



First Epidermis 

 RBC's thickens 



Many 

 RBC s 



Cutaneous respiration 



Superficial 

 1 layer 



Transition, 

 "?0<R'e'<2°° 



Functional gut 



Movable lower Jaw 



2.5 days 



:t 



I I 

 5 



Gill respiration 



Scale formation 



Midline 



2-3 layers 3-4 layers 



7-8 layers 



Re>200 



Stomach forms 

 .It- 



Expandable mouth 

 3.3 days 4 days 



T^ 1 1 1 — I 1 1 1 1 1 1 1 1 1 1 1 — 1- 



15 20 25 30 



Length (mm) 



Juvenile 

 period 



Filler feeding 



15 days 



* 



1 — I 1 1 1 1 1 — 



2.5 5 10 15 20 25 30 



35 40 45 



Days at 16° C 



— 1 1 1 1 1 1 1 — 



50 55 60 65 70 75 80 



Fig. 2. Events during development of the northern anchovy. RBC = red blood cells. Time to 50% starvation is number of days to starvation 

 at which 50% of the fish died (from Hunter and Coyne. 1982). 



Much of this work is summarised by Theilacker and Dorsey 

 (1980). 



Over the past few years the assembly of much basic data has 

 allowed the current vogue for modelling to be applied to fish 

 larvae. Modelling is an attempt to synthesise and simplify basic 

 data usually in mathematical form. Mathematical models are 

 often iterative and they have the value of being in a form suitable 

 for computers. Laurence (1981) has recently reviewed modelling 

 work on fish larvae and the complexity and type of interaction 

 is shown in Fig. 3. The main problem addressed has been that 

 of feeding. The earlier models of Blaxter (1966), Rosenthal and 

 Hempel ( 1 970), Blaxter and Staines ( 1 97 1 ) and Hunter (1972) 

 estimated the feeding efficiency of larvae, the volume of water 

 searched in unit time and the density of food required to give 

 good survival and growth. More sophisticated models have now 

 been developed (e.g., Jones and Hall, 1 974; Beyer and Laurence, 

 1981) and Vlymen's (1977) model allows for the prey species 

 being non-randomly distributed. 



The need for larvae and their prey to co-exist temporally was 

 spelled out by Gushing ( 1 975) in his match-mismatch hypothesis. 

 Thus the timing of reproduction appears to have evolved to 

 synchronise the larval stages with the main phase of the annual 

 production cycle. Spawning is probably controlled in most tem- 

 perate fish species by photoperiod and temperature which are 

 not the only determinants of plankton production. Hence a 

 match or mismatch is possible between this production and the 

 presence of fish larvae with a resulting influence on year class 

 strength. 



An early paradox existed in that the density of the larger 

 micro-zooplankton such as copepod nauplii required for good 

 growth and survival in tanks was of the order of 1 organism/ 

 ml. Such densities are rarely found in the sea as judged from 

 normal plankton sampling. This led to the suggestion of micro- 

 scale patchiness of food in the sea, which might occur at inter- 

 faces such as steep thermoclines and at tide- and wind-induced 

 fronts. The integrity of such microscale patchiness would not, 

 of course, be obvious using nets sampling large volumes. 



This led Lasker (1975) to bioassay samples of water taken at 

 different depths and places off the Califomian coast, using an- 

 chovy larvae both hatched and tested on board ship. Chloro- 

 phyll-rich layers with very high densities oi Gymnodinium were 

 found near the thermocline. The bioassay showed good larval 

 feeding in these water samples, suggesting that patchiness, in- 

 deed, might be a valid concept. This was to some extent con- 

 firmed by later findings that stable weather conditions (which 

 maintained the thermocline) favoured good year classes of an- 

 chovy larvae off the Califomian coast (Lasker, 1981). Owen 

 ( 1 980) has subsequently shown from samples taken by plankton 

 pumps and water bottles that patchiness of microzooplankton 

 such as copepod nauplii and tintinnids and various protozoan 

 species and phytoplankton (some of which are known to be the 

 food of anchovy larvae) exist off the Peruvian and Califomian 

 coasts on the scale of a few centimetres up to one metre (see 

 Fig. 4). Only Houde and Schekter ( 1978) have attempted to rear 

 larvae in simulated food patches and found that survival of sea 

 bream was similar when they were exposed to 3 h of food per 



