GENERAL PREMISES AND SELECTED ELEMENTS OF A METHOD OF ESTIMATING FISH STOCKS AND POPULATIONS IN POLISH LAKES
PREMISSES GENERALES ET ELEMENTS SELECTIONNES D'UNE METHODE D'EVALUATION DE STOCKS ET DE POPULATIONS DE POISSONS DES LACS POLONAIS

by/par

M. Leopold and B. Dabrowski

Department of Fishery Economics,
Inland Fisheries Institute, Olsztyn
Poland/Pologne

ABSTRACT

Poland has many inland waters which support a variety of fish populations forming the basis for commercial as well as sport fisheries. The intensity of exploitation is estimated by comparing the performance of different fishing gears in the various bodies of water and a range of other relationships can also be derived from this information.

RESUME

La Pologne a beaucoup d'eaux intérieures abritant une grande variété de populations de poissons dont dérivent ses pêches commerciale et sportive. L'intensité d'exploitation est estimée en comparant l'efficacité des différents engins de pêche dans les divers plans d'eau et une gamme d'autres rapports peuvent également être extraits de ces informations.

CONTENTS

1. GENERAL PREMISES

1.1 Characteristics of Polish Lakes

1.2 Characteristics of Fish Stocks

1.3 Characteristics of Fishery Exploitation

1.4 Estimation of the Intensity of Fishery Exploitation

1.5 General Evaluation of Fish Stocks and Particular Fish Populations

1.6 Relation: Catches, Fishery Exploitation

1. GENERAL PREMISES

1.1 Characteristics of Polish Lakes

There are about 500 000 ha of Polish inland waters, 70 percent of this area being lakes on which fishery management is carried out, either for production or for recreation, but most frequently for both.

Polish lakes are generally small; 90 percent of the total number and 32 percent of the total area are lakes of up to 100 ha; lakes with an area 100–500 ha cover 31 percent of the total and lakes bigger than 500 ha occupy 37 percent of the total area of lakes. The lakes are relatively shallow as 70 percent of their number are less than 20 m deep and, of these, 50 percent are less than 10 m deep.

According to limnological typology, most belong to the eutrophic type (51.6 percent); oligotrophic and mesotrophic lakes amounting to 26.4 percent. The remainder are mainly “pond” type (20.5 percent) and 1.5 percent are dystrophic. The percentage composition of lake types based on existing fish species composition and potential possibilities of its management, and also on some chosen elements of the environment (for example depth), is as follows: vendace type 33 percent; bream type 25 percent; pike/perch type 19 percent; tench and pike type 21 percent; and Crucian carp type 2 percent.

Polish lakes are mostly concentrated in three lake districts, constituting in some regions aggregations amounting to 10 percent of the total area. This, of course, makes fishery management of lakes much easier. Apart from exceptional cases of coastal lakes, almost all Polish lakes are of post-glacial origin, among which prevail gully lakes.

1.2 Characteristics of Fish Stocks

Although high differentiation of fish species composition in various lakes decreases the value of all general means, they still give at least general knowledge on the character of productional basis, ichthyofauna and, to some extent, on fishery management. Average commercial catches in kg/ha and percent during the period 1960–70 are given in Table I.

Angling catches amount to about 30 percent of commercial catches and consist mostly of roach, perch, pike, eel, and bream. The ichthyofauna of Polish lakes, apart from a general lack of Salmonidae species (excepting coregonids), is characterized by highly differentiated fish stocks composed of: predatory and non-predatory species, plankton and benthos feeding, with different spawning periods, inhabiting various zones, occurring in shoals and dispersed, etc.

In a given fishery and production situation various fish species are directly or indirectly either valuable or undesirable. All absolute appraisal of their value is nonsense.

In order to preserve one of the main values of inland waters, i.e., their fishery productivity, it is necessary to manage them rationally and a basic condition for such management is a possibility of obtaining quick and reliable information on the general state of the fish stock in a given lake, the state of particular fish populations, and changes taking place in the ichthyofauna. Fishery exploitation, in the broad sense, is one of the main elements which makes a rational management of lakes possible and, at the same time, provides necessary information on a given body of water.

1.3 Characteristics of Fishery Exploitation

Differentiation and variability of fishery exploitation, carried out with different methods and fishing gear, is a natural consequence of the differentiation of fish stocks in lakes, and their relative value whether positive or negative.

Catches are carried out with about 30 different types of gear including various more or less important modifications of any given gear. Although some fishing gears presented here play a decisive role on particular lakes, on average they constitute about 90 percent of the total fishing intensity in which summer and winter seines constitute above 50 percent.

Chatacterizing fishery exploitation of Polish lakes from another point of view, it may be stated that one fisherman exploits 220 ha of lake surface. Excluding all holidays and periods when fishing is impossible (melting of ice cover, etc.), it may be stated that during one day of work one fisherman fishes one ha of water area. In this context, the role of fishery exploitation becomes unquestionable.

1.4 Estimation of the Intensity of Fishery Exploitation

It is a basic assumption of the method presented that fish caught per unit of fishing intensity (catch per unit effort), or any other such index, is a measure of fish stock density, or density of populations of various fish species. With such an assumption, it is necessary to evaluate this intensity exactly and independently for the different types of fishing gear used for catches. This necessity, apart from the fact that catches as a rule include several fish species, constitutes a fundamental difficulty of the method.

A conventional fishing gear, catching one kg of fish daily, was taken as a standard unit of fishing effort for comparison. Then, based upon long-term catch data from different types of fishing gear and collected in various parts of Poland from hundreds of lakes, the average daily catch of the different types of fishing gear, expressed in kg of fish per day, was calculated. The values thus obtained were considered as standard fishing effort for each fishing gear. Thus, the fishing intensity of a given fishing gear is the product of its standard fishing effort multiplied by the number of days during which it is used. Subsequently, the catch per unit of area, or intensity per unit of area (standard units per ha), can be derived.

Thus, when the method is properly applied, the only element necessary for an estimation of general intensity of exploitation is the number of days during which catches were carried out with a given fishing gear. This value is obtained from records maintained in fish farms, directly by the fishermen.

Such an approach, in its simplest form, is however valid only for total fish stocks. In the case of populations of individual species, the whole matter becomes more complicated since different types of fishing gear are characterized by different selectivity in respect to various fish species. As a result, in calculating intensity of catches of particular fish species, it is necessary to take into account the selectivity of different fishing gears. For this purpose, the average percentage share of various species, in catches carried out with different types of fishing gear, was calculated for Polish lakes. Calculation allowed for an estimation of selectivity of fishing gear in respect to different fish species.

1.5 General Evaluation of Fish Stocks and Particular Fish Populations

The total fish catch on a given lake, divided by the sum of the fishing intensity of all types of fishing gear, gives an index as discussed in 1.4. The index allows for carrying out proper evaluation of the state of fish stock in a given lake. In Poland it is usually called the “index of effectiveness of exploitation”. Used in the simplest manner, it gives a relative measure of the density of fish stock and also, indirectly, its number. Estimation of number in a stock or population of a given species of fish is only possible almost exclusively by rather complicated scientific studies. However, considering possibilities and particularly the frequent necessity of making quick decisions as regards directing the management of fish stocks, such methods are of little value.

The index of effectiveness of exploitation is a satisfactory measure of the density (or number) of fish in a given lake, providing that:

1. fish in a given reservoir are distributed evenly, i.e., their density is the same all over the reservoir;

2. their catchability, i.e., susceptibility to catching, is similar in comparable samples.

Normally, fish in a lake are not distributed evenly. Due to the possibility of free movement, they gather in some areas and in certain periods. Thus, variability of the density and catchability of fish should be included in any analysis of an index of effectiveness of population as a measure of fish number. The greatest changes are observed in different seasons and, because of this, material can be compared without any reservations during an analysis, providing that they are numerous enough to be representative for a whole year, or that they represent the same season and conditions. A comparison of data from the same reservoir is most reliable and such data allow further conclusions to be drawn. Some phenomena are connected with similar reservoirs; for example, fishing efficiency of different types of fishing gear depends on the size of a reservoir.

The average daily catch of different types of fishing gear is essentially analogous to an index of effectiveness of exploitation. Depending on the more or less pronounced selectivity of various fishing gear, these can form either an index of the density of a whole fish stock or - and in this case their value becomes exceptionally high - the density of populations of particular fish species.

1.6 Relation: Catches, Fishery Exploitation

Without going into details, it may be stated that long-term studies have shown that there exist statistically measurable and calculable relationships occurring in two groups, as follows:

1. Relationships concerning fish stock:

   • total intensity of exploitation; total catch; index of effectiveness of exploitation (usually in standard units per ha, kg/ha, and kg per standard unit);

2. Relationships concerning populations of particular fish species:

   • fishing intensity of a gear; catch of a given species; average daily catch of a gear (usually in standard units per ha, kg/ha, and kg/day).

The above relationships exist also between different parameters of the groups (i) and (ii), or their respective transformations. For example, catch of a given species in kg per standard unit, fishing intensity in respect to a given species in standard units per ha, etc.

Utilization of the above relationships allows for several substantial determinations, namely:

• estimation of the density of fish stock or populations of particular species in a given reservoir, as compared to other reservoirs or to any other relevant item;

• estimation of changes in the density of stocks or populations of particular fish species, in comparison to their state at any other time as a standard point.

Such determinations may be carried out for one reservoir or for lake complexes, lake districts or even different countries.

Application of the method to lakes on which systematic commercial exploitation is not carried out is certainly more difficult, but not impossible. It is possible to obtain respective parameters for the above relationships based upon established statistically reliable methods of sampling catches.

As concerns preparation of material necessary for proper determinations and conclusions, the method under discussion does require more than elementary statistics, especially analysis of correlation and regression.