REVIEW
EXPOSE

by/par

K. Tiews
Bundesforschungsanstalt für Fischerei
Palmaille 9, 2 Hamburg 50
Federal Republic of Germany/République fédérale d'Allemagne

ABSTRACT

Sampling for abundance of eggs, larvae and juveniles of fish is not widely used in fresh water although methodology is well defined for marine stocks. In some fisheries stock size can be estimated from egg production, but in general the unpredictable mortality between eggs and juvenile fish makes such estimates unreliable and the juvenile stage is a better basis for such calculations. Many methods have been used for the capture of the early life stages of freshwater fish and there is a wide field for future research on this topic.

RESUME

L'échantillonnage servant à déterminer l'abondance des oeufs, des larves et des jeunes poissons n'est guère pratiqué en eaux douces alors que la méthode est bien définie pour les stocks marins. Dans certaines pêcheries, il est possible de déterminer l'ampleur du stock à partir de la production d'oeufs. La mortalité imprévisible des oeufs et des jeunes poissons rend toutefois ces estimations généralment peu fiables et le stade juvénile constitute une meilleure base pours de tels calculs. De nombreuses méthodes ont été utilisées pour la capture des premiers stades du cycle biologique des poissons d'eau douce et de nombreuses recherches ultérieures sont possibles dans ce domaine.

Looking through the freshwater fisheries literature, it is striking to observe that little attention has been paid to the quantitative sampling of fish eggs, larvae and juvenile fish despite the fact that immense knowledge on this subject does exist in the field of marine fisheries research, and a group of experts in this field has recently prepared a document on this subject (Hempel, 1973).

In marine fisheries research the sampling of eggs, larvae and juvenile fish aims generally at the detection and appraisal of fishery resources and at studies on the population dynamics of fishes. Surveys of this type have been found suitable to explore for new resources, to locate spawning concentrations of important stocks, to describe relative abundances of commercially important stocks including comparison within and amongst regions, to monitor long-term changes in the composition and abundance of resources, and in spawning times and areas. In this field of research modern techniques of sonic surveys as well as data from exploratory fishing and from commercial fisheries play a dominant role. Abundance estimates of fish eggs and larvae provide additional independent information on the abundance of particular fish species. But such surveys have also been made for tracing fluctuations in spawning stocks by estimating the abundance of their eggs and young larvae, to forecast year-class strength on the basis of the abundance of older larvae, to estimate abundance of stock based on its spawning production and to discriminate between stocks of the same species. Ichthyoplankton surveys have been successfully applied to accomplish all these objectives, but there are also many references demonstrating the difficulties in the interpretation of such data or even the failure of such operations. Ichthyoplankton surveys are certainly not the sole method in this respect, but they usually compete with other surveys methods as well as with the analysis of fisheries statistics. In marine fisheries it has been found that catch and effort data have become unreliable in many fisheries because of rapid changes in the operations of modern fishing fleets. Therefore, research workers must apply other methods to get accurate estimates of stock size. Ichthyoplankton surveys have proved in several cases a reliable measure to determine stock size. The importance of basing population assessments and management advice on more than one independent set of evidence is generally recognized.

In some fisheries the stock size has been successfully estimated from the spawning production, i.e., the number of eggs in the spawning area, in combination with the knowledge on the fecundity of the fish at different ages as well as on the age composition of the spawning stocks.

The following kinds of information must be obtained:

1. Developmental series of target species from eggs and larvae through to early juvenile stage;

2. Rate of development of eggs (and larvae) as related to water temperature;

3. Depth distribution and horizontal patchiness of eggs and larvae of target species under varying oceanographic conditions and at day and night;

4. Eggs production of spawning females as related to weight and possibly to age (fecundity studies). In most cases fecundity per unit of body weight, and weight-length and age-length keys will be sufficient to relate egg production to stock biomass;

5. Structure of spawning population - ratio of males to females as related to age, the weight and size relation between males and females, etc.;

6. Sub-populations or stocks comprising the population, if more than one.

In several of the fish species it has been found that a forecasting of year-class strength as a mean for the forecasting of the development of the fisheries, is not possible on the basis of the abundance of eggs and young larvae, but is possible on the basis of the abundance of older larvae. In other fish species the natural mortality even of older larvae and of juvenile fish during their first few months of life is still much too unpredictable so that abundance indices of these cannot be used for a forecast of the year-class strength. But in many of the important marine commercial fish species, it has been found that juvenile fish after 4–6 months of age do usually not suffer any greater natural mortality so that abundance indices of these age groups can excellently be used to predict the strength of a year-class. For this reason many of the marine fisheries research institutes have included such young fish surveys in their research programmes on a routine basis. Sampling gear are here mostly trawl nets, both bottom and pelagic ones.

The usefulness of ichthyoplankton surveys in marine fisheries research is very much restricted to the cooler climate zones. In the warmer climate zones the development of eggs can be so fast that quantitative ichthyoplankton surveys can become more or less meaningless. In tropical waters for example larvae hatch within 24–36 hours.

There are quite a few descriptions of sampling techniques to carry out fish eggs and larvae surveys. A comprehensive report has been given by Nellen and Schnack (see p. 538) Hempel and the members of his study group recommend for quantitative collections of fish eggs and larvae on survey cruises the use of two different meshes in the paired Bongo nets.

Freshwater fisheries research, as has been said above, has paid until now little attention to the quantitative sampling of fish eggs, larvae and juvenile fish. The reason for the general lack of interest in quantitative sampling of eggs, larvae and juveniles of fresh- water fish, is certainly that nearly all species do not have pelagic eggs, and that larvae and juveniles often stay in sheltered coastal areas between underwater plants and are therefore difficult to sample. Cyprinidae and Percidae for example produce adhesive eggs, which are fastened to underwater plants; Salmonidae dig spawning holes and bury their eggs into the sediment; and Coregonidae spawn their eggs into the free water where they sink and remain close to the bottom. Nellen and Schnack give further examples of such semi-pelagic eggs.

Raymond and Collins (see p. 552) describe techniques for the appraisal of migrating juvenile anadromous fish populations in the Columbia River basin. In free flowing rivers they used self-cleaning scoop traps and in impoundments and sections of the river, in which velocities are minimal, “Mervin Traps”, gillnets, beach seines and purse seines have proven effective. At dams the authors collected juvenile salmon and trout in gate-wells in large numbers by means of a specially designed dipnet using the dam as a collector.

High-speed Miller samplers used by Forney (see p. 581) have proved satisfactory for sampling larval walleyes during their pelagic stage in Lake Oneida.

Thorne, Dawson, Traynor and Burgner (see p. 328) studied the population of juvenile sockeye salmon in Lake Washington with the use of acoustic assessment techniques and by means of a 3-m Isaacs-Kidd midwater trawl. The acoustic survey typically consisted of a number of transeots distributed over the lake. The survey was conducted at night, when fish were dispersed in midwater. The analysis of data is described in the paper. The fish population estimates derived from the net hauls, and the acoustic surveys were found to be similar. Seasonal population estimates provided valuable data for evaluation of rates and possible causes of mortality during lake residence. The population levels prior to out-migration are of particular value for fishery management as an index to the number of returning adults. The authors state that in many respects the fishery resource in Lake Washington is ideally suited to acoustic assessment techniques. The basic hydro-acoustic techniques developed and tested on the stock of juvenile sockeye salmon in Lake Washington are now being applied to the assessment of limnetic fish in several sockeye salmon-producing lake systems in Washington, British Columbia and Alaska, as well as in several lake systems where sockeye salmon are not present. The main limitation in these applications concerns species identification. They provide estimates of number, but not kind. Therefore, considerable effort is being expended on acoustic methods of size determination to aid identification.

Davies (see p. 236) demonstrated the validity of rotenone sampling in reservoirs in Brazil. The variability between samples for various types of data and formulae for determining sample size are given. Although he can assess the number of young fish of a variety of different species, he states that the method is less useful in determining their relative strength of year classes. Hall (see p. 249) also gives a complete review on the experiences made in sampling reservoir fish populations with rotenone.

Nellen and Schnack (see p. 538) review sampling problems and methods of fish egg and larvae investigations with special reference to inland waters. They also give results of survey work in several inland waters of Schleswig-Holstein. Fishing was done by means of a plankton net, towed at a speed of 1 knot just below the surface. Good catches of smelt larvae and perch larvae and young sticklebacks were caught, but practically no Cyprinid fish larvae. In another experiment a neuston net survey has been performed in the upper part of the Eider for the study of the horizontal distribution of fish larvae in relation to the wind drift of surface water layers, at which considerable numbers of larvae of Osmerus eperlanus (6–11 mm); Acerina cernua (5–7 mm) and Cyprinids (Abramis brama and Rutilus rutilus 7–11 mm long), were caught. Cyprinid larvae were missing completely in the waterway tows.

The authors state that sampling techniques for larvae used in the marine environment may be applied in the case of Coregonid larvae surveys because the fry of whitefish can be expected to inhabit the epipelagic zone just as the adults do. A general problem is the lack of powerful research boats to handle a Gulf III-sampler or a Bongo net in freshwater research but pair trawling with smaller boats might help. The authors also review other larvae sampling gear, such as an arrangement for sampling semi-buoyant eggs in a stream, a surface plankton sampling device, the Miller high speed sampler. Because of the small size (10 cm mouth aperture) of the latter it can be operated from a small boat powered by outboard motor and it also allows sampling of deeper water layers. Other gears mentioned are purse seines, and the use of electric shockers in combination with the Miller sampler.

In general, it can be concluded that there is a wide field for future research on the ecology and behaviour of eggs, larvae and juvenile fish for each particular fish species in our inland waters. Freshwater fisheries biologists have to learn from the marine fisheries biologists that there is no inexpensive way for monitoring these three life stages of fish quantitatively (daily costs for ocean-going research vessels amount to 15 000–20 000 German marks). In comparison to such costs even very much stratified sampling programmes in inland waters should be cheaper.