Piscator publications (with abstracts)

• Lammens, E.H.R.R. 1999a, Modellering vispopulaties. In: Lammens, E.H.R.R. Het voedselweb van het IJsselmeer en Markermeer - Veldgegevens, hypotheses, modellen en scenario's [in Dutch]. RIZA report 99.008, ISBN 903695228X pp 123-139.
  
  
  
• Lammens, E.H.R.R., 1999b. The central role of fish in lake restoration and management. Hydrobiologia 396: 191-198.
  Abstract:The central role of fish in lake restoration and management has a practical purpose: fish are much easier to manipulate than nutrients, phytoplankton and zooplankton, and therefore they are a relatively easy (additional) instrument in restoration and management. The management of the fish stock may be a measure of water quality, of fish stock composition or a measure of both and may vary from very drastic removal of planktivorous and benthivorous fish to a more gradual change in the population by continual predator management and less drastic reduction of inedible prey. For lake restoration, drastic removal is the most efficient in order to obtain clear water and vegetation and a subsequent fish community adapted to this. Continual management will result in a more gradual change and may be more acceptable to the interest of both fishermen and water quality managers
  
  
  
• Lammens, E.H.R.R., E.H. Van Nes, & W.M. Mooij, accepted. The development and exploitation of the bream population in three lakes in relation to water quality. Freshw.Biol.
  Abstract:
  1. The development of the bream population is analysed in three differently exploited lakes: the Frisian lakes (10.000 ha), Lake Veluwe (3000 ha) and Lake Volkerak (4500 ha). A model is used to analyse the effect of fishery on the population. Apart from direct effects on the bream population indirect effects of fishery on transparency, chlorophyll-a, vegetation and macrofauna are analysed.
  2. In Lake Veluwe the bream population was reduced from ca. 100 to 20 kg ha^-1 after five years of fishing. The mortality caused by the fishery was estimated 62 % of bream > 15 cm on top of the 15 % natural mortality for bream > 8 cm. The collapse of the bream population was followed by a clearing of the lake and a rapid expansion of the Chara-beds present in the shallow parts. The increase of the Chara-beds occurred simultaneously with the increase in transparency in the open water, the explosion of zebra mussels and decline of chlorophyll-a.
  3. The newly created Lake Volkerak showed the opposite to what happened in Lake Veluwe. The unexploited bream population started to develop in 1988 and reached a biomass of ca. 140 kg ha^-1 in 1998. The transparency in the lake decreased from a maximum of 3 m to ca. 1 m. In the same period the chlorophyll-a level increased 5 mg l^-1 of 45 mg l^-1. Vegetation started to develop in the first years to 20 % of the total area but decreased to 10 % with increasing turbidity.
  4. The seine fishery in the Frisian lakes did not seem to affect the bream population despite high catches of 40-50 kg ha^-1. The calculated natural mortality of fish larger than 12 cm was ca. 25 %, whereas the fishery mortality added 46 %. The high loss was compensated by good recruitment and higher growth rates because of the higher temperatures in the period of fishing. There was only a slight decrease of chlorophyll-a and slight increase of transparency.
  
  
  
  
  
  
• Lammens, E.H.R.R., M.L. Meijer, E.H. Van Nes, & M.S. Van den Berg, submitted. The effect of commercial fishery on the bream population and relation to the rapid expansion of Chara beds in Lake Veluwe.
  Abstract:
  1. The fish community in Lake Veluwe (3400 ha) changed in composition and biomass, simultaneously with the development of the Chara beds. The number of fish species increased considerably and the total biomass decreased to ca. 35% of the original biomass. The change in the fish biomass followed the introduction of a commercial fishery and was in its turn followed by a rapid expansion of the Chara beds.
  2. A fish model was used to describe the development of the bream population in Lake Veluwemeer before and after the commercial fishery started. We calculated the natural mortality using data on growth, recruitment and size distribution and discriminated between natural and fishery mortality. The model predicted that without fishery the biomass of the bream population would hardly change and that the fishery could explain the change in the biomass and size composition.
  3. The rapid expansion of the Chara beds was not likely without the fishery as the Chara needs transparency up to the bottom and undisturbed sediment. The reduction of the bream population created favourable conditions for zebra mussels to settle, which in turn created the clear water, necessary for the further development of the Chara beds.
  
  
  
  
• van Nes, E.H., 1999. PISCATOR 2.0 Model description. Internal Report. Download pdf Document.
  
  
  
• van Nes, E.H., E.H.R.R. Lammens & Marten Scheffer 2002. PISCATOR, an individual-based model to analyze the dynamics of lake fish communities. Ecol. Modell. 152: 261-278.
  Abstract:Unraveling the mechanisms that drive dynamics of multi-species fish communities is notoriously difficult. Not only the interactions between fish populations are complex, also the functional niche of individual animals changes profoundly as they grow, making variation in size within populations and even within cohorts highly important to consider. Not surprisingly, traditional aggregated populations models have proved to be limited in their capacity to describe dynamics of interacting fish species, and individual-based models have become popular for modeling fish populations. Nonetheless, the majority of the individual-based models describes either a single species or focus entirely on a certain life stage. We present the individual-based model Piscator, which describes a multi-species fish community, and demonstrate techniques to deal with the inherent complexity of such a model. We propose a novel procedure for calibration and analysis in which the complexity of the model is increased step-by-step. We also illustrate the use of a special Monte-Carlo sensitivity analysis to identify clusters of parameters that have roughly the same effects on the model results. As an example we use the model to analyze a fishery experiment in the Frisian Lakes (The Netherlands). Despite of high bream catches (40-50 kg ha-1yr-1), it was observed that the seine fishery had unexpected little effect on the bream population. Our simulation results suggest that if one takes community feedbacks and climatic variability into account, this effect can be explained. The main cause was, besides a reduction of piscivory due to a simultaneous gill-net fishery, a coincidental strong year-class just before the fishery started. The strong development of this year-class could be explained by three subsequent warm years, whereas yearly variations in recruitment were less important.
  We also suggest that this relatively realistic model could play a role in ecological theory. It can be used to analyze the conditions for multi-year cycles and chaotic dynamics, phenomena that are usually predicted only from simple abstract models.
  
  
  
• van Nes, E.H. 2002. Controlling complexity in individual-based models of aquatic vegetation and fish communities. PhD Thesis. Wageningen University (will be available as pdf)