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Sized-based tools for managing freshwater fish communities

Objective

- To implement a monitoring programme for fish communities based on the biomass size distributions of the fish and their resources, and on the biomass size distribution of the phytoplankton community in an environmental gradient over a wide geographical range in Europe.
- To analyse factors determining fish community size distribution within different habitats, relating present size distributions and size related characteristics such as growth, maturation/fecundity and mortality to the resource size distribution and environmental variation.
- To derive size-dependent competitive ability relationships of major key species on different prey resources along the productivity gradient of European lakes.
- To assess the consequences of different size selective harvesting strategies for inter- and intraspecific competition and hence for the structure and the dynamics of the fish communities along a productivity gradient of lakes.
- To examine 'causes for the generation of size variation among individuals, especially in early stages of life by using individual-based modelling and experiments in controlled systems and to analyse how the scaling of foraging capacity to body size affects the likelihood of cohort slitting.

The project was started 1. April 1997 and will last for four years ending 1. April 2001. As the project is newly started, few results are yet available. This article summarizing the planned work and dissemination activities.
DISSEMINATION ACTIVITIES
Two project meetings have been arranged in 1997, one in March and one in October. The kick-off meeting in March discussed the research strategies and made decisions on methodology. In the October meeting all partners presented results from the first year, which were analysed at the task level. The partners have to variable extent made the project known for management authorities on a national basis.

During a scientific visit to USA in September 1997, the project was presented by the Coordinator at The University of Wisconsin at two places; Centre of Limnology in Madison and Centre for Great Lakes Research in Milwaukee.

During a workshop in May 1997 at Cornell Biological Field Station, New York, United States, Wim van Densen presented the project with the contribution "Percid dynamics".

The project was also presented at a workshop on "Aquatic ecological instruments for the future", Utrecht, The Netherlands, December 1997 by Joep de Leeuw.
METHODOLOGY AND RESEARCH TASKS

The overall methodology relates to three interrelated components: analyses and monitoring of field communities, experiments (aquarium, pond) of size/individual-dependent ecological capacities and theoretical modelling of population dynamics and individual behaviour. For the first component, size spectra analyses and univariate multivariate analyses will be used. The second and third components focus on size class variation combined with population models specially designed to handle size-based demography and individual variation combined with game theoretical models. The project consists of the following three research tasks:

Task 1: The monitoring approach.
Subtask 1.1: The role of environmental variation on fish community and species size distribution
Subtask 1.2: Factors influencing size-dependent growth and mortality

Task 2: The inductive approach.
Subtask 2.1: Size-dependent competitive and predator-prey relationships
Subtask 2.2: Modelling of size-structured fish populations along a productivity gradient

Task 3: The game theory approach to size variation
Sub-task 3.1: Game-theory modelling approach to variation in body size
Sub-task 3.2: Empirical tests of size variation

Objectives:

The objectives of task 1 are to (1) implement and assess monitoring programmes for fish populations and communities based on the biomass size distributions of the fish, their resources, and the biomass size distribution of the phytoplankton community in a series of lakes in an environmental gradient over a wide geographical range in Europe and (2) analyse factors determining fish community size distribution within different habitats, relating present size distributions and size related characteristics such as growth, maturity/fecundity and mortality to the resource size distribution and environmental variation.

METHODOLOGY:

Most of the extensive data sets necessary for the characterization of lake communities are already in the possession of the partnership, additional information will be collected by monitoring programmes performed during the research period. Biomass size spectra will be constructed by assigning all organisms to size classes according to their individual biomass and computing the total biomass per size class. Seasonal changes in the spatial distributions will be accounted for to guarantee the representativeness of such overall size distributions. Biomass size spectra will be calculated and analyzed using different statistical techniques (e.g. regression analysis). This analysis will be based upon species as well as on functional (trophic) groups. Age and catch curve analysis will be employed to generate estimates for size-specific growth and mortality rates in fish populations. These estimates will be combined with data on somatic and reproductive allocations. Individual growth trajectories will be estimated using back-calculation for individual specimen, and changes in growth rates will be analysed in relation to the individual sizes of the fish, the fish densities, the species composition (and functional groups), the available size distribution of the food resources and the environmental variation. Inter-annual variations in the recruitment in YOY fish will be analysed using results from surveys of young fish, whereas recruitment patterns will be reconstructed from the age composition of the adult part of the population.

Task 2: The inductive approach.

OBJECTIVES:

The objectives of task 2 are (1) to derive size-dependent competitive ability relationships of major key species on different prey resources along the productivity gradient of European lakes and (2) to assess the consequences of different size selective harvesting strategies for inter- and intraspecific competition and hence for the structure and the dynamics of the fish communities along a productivity gradient of lakes. The first part involves the development of experimental data which, besides being used to assess competitive abilities, will be implemented into physiologically structured models developed under the second part with the ultimate purpose to assess the consequences of different harvesting regimes. The harvesting of fish involves two aspects: the intensity (effort) and the size selectivity (mesh size used) of harvesting. The best harvesting strategy to be used, i.e. how much and which size classes of the piscivore and the prey fish that should be harvested, will vary depending on the type of piscivore and prey species present.

METHODOLOGY:

The basic set up of this task relates to documented changes in European fish communities along the productivity gradient of freshwater lakes. The fish assemblages typical of the endpoints and the middle of the productivity gradient (low productive mountain lakes, intermediate productive forest lakes and highly productive lowland lakes) have been selected. The target species will in all three cases involve a piscivore and a planktivore/benthivore being both a prey and a competitor to the piscivore. For each of the 3 productivity constellations, the choice of the species pair is based on that (1) previous field and experimental studies have shown them to be key interactors in the specific communities and (2) they show the same type of basic interactions with respect to a piscivore undergoing ontogenetic niche shifts with body size from zooplanktivory, via feeding on macrofauna elements to piscivory. The species constellations selected are brown trout and Arctic char (low productive mountain lakes), perch and roach (medium productive forest lakes), and pikeperch and bream (highly productive lowland lakes), respectively. That the best harvesting regime differs between piscivores relates to the different life histories of the key piscivores with brown trout shifting to piscivory at a larger size (later), perch at an intermediate size, and pikeperch at a smaller size (earlier). As a consequence, these piscivores differ in the degree by which they (1) suffer from competition as juveniles, and (2) are able to utilise and control prey fish. Task 2 is based on a combination of experiments at different scales (aquarium, ponds), lake field data and modelling.

Task 3: The game theory approach to size variation

Objectives:
In this section of the project the main objective is to uncover (1) why size variation among fish individuals emerges in the first place, and (2) why it is maintained and often enhanced in natural populations. Variation in size is likely to produce differences in the behavioural strategies that individuals will adopt. The scope is to derive models to predict the growth of individual fish in the population. By using the models, testable predictions about the causes yielding size variation in growing fish are made. These will be tested in laboratory and hatchery experiments and compared against data available from natural populations. Findings of this research will then facilitate understanding the results from the other tasks, the monitoring (task 1) and inductive (task 2) approaches.

Methodology:
The study combines (i) theoretical modelling, (ii) small-scale experimentation in the laboratory, and (iii) experimentation in fish hatcheries and in field conditions. The modelling process begins with simple constructions and gradually builds up towards models acknowledging more and more information about the characteristics of the ambient environment (temperature, food availability, predation risk). In the empirical part origin and development of size variation are studied in detail from the egg stage throughout the ontogenetic development and growth of the fish. The significance of genetic and environmental factors in determining fish size will be scrutinized experimentally. In the tests four of the key piscivore species of this project are used as target species: Arctic char, brown trout, pike-perch and perch.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Norwegian University of Science and Technology
Address
Bjornsons Gt 12
7043 Trondheim
Norway

Participants (7)

AGRICULTURAL UNIVERSITY OF WAGENINGEN
Netherlands
Address
Marijkeweg 40
Wageningen
National Board of Fisheries
Sweden
Address
Stangholmsv
178 93 Drottiningholm
ROYAL NETHERLANDS ACADEMY OF ARTS AND SCIENCE
Netherlands
Address
6,Rijksstraatweg 6
3631 AC Nieuwersluis
UNIVERSITY OF HELSINKI
Finland
Address
Yliopistonkatu 4
00014 Helsinki
Umea University
Sweden
Address
Linneaus Vag
901 87 Umea
Universiteit Amsterdam
Netherlands
Address
Kruislaan 320
1098 SM Amsterdam
University of Lund
Sweden
Address
5,Ecology Building
223 62 Lund