Modelling the effects of fishing on the North Sea fish community size composition (original) (raw)
Related papers
Using size-based indicators to evaluate the ecosystem effects of fishing
ICES Journal of Marine Science, 2005
The usefulness and relevance of size-based indicators (SBIs) to an ecosystem approach to fisheries (EAF) are assessed through a review of empirical and modelling studies. SBIs are tabulated along with their definitions, data requirements, potential biases, availability of time-series, and expected directions of change in response to fishing pressure. They include mean length in a population, mean length in a community, mean maximum length in a community, and the slope and intercept of size spectra. Most SBIs can be derived from fairly standard survey data on length frequencies, without the need for elaborate models. Possible fishing-and environment-induced effects are analysed to distinguish between the two causes, and hypothetical cases of reference directions of change are tabulated. We conclude that no single SBI can serve as an effective overall indicator of heavy fishing pressure. Rather, suites of SBI should be selected, and reference directions may be more useful than reference points. Further modelling and worldwide comparative studies are needed to provide better understanding of SBIs and the factors affecting them. The slow response to fishing pressure reflects the complexity of community interactions and ecosystem responses, and prohibits their application in the context of short-term (annual) tactical fisheries management. However, movement towards longer-term (5-10 years) strategic management in EAF should facilitate their use. [Many authors in the volume are using full first Christian names, so I've added those I know. Can you add yours] Shin, Y-J., Rochet, M-J., Jennings, S., Field, J. G., and Gislason, H. 2005. Using size-based indicators to evaluate the ecosystem effects of fishing. -ICES Journal of Marine Science, 62: 000-000.
Journal of Applied Ecology, 2014
1. Marine environmental management policies seek to ensure that fishing impacts on fished populations and other components of the ecosystem are sustainable, to simultaneously meet objectives for fisheries and conservation. For example, in Europe, targets for (i) biodiversity, (ii) food web structure as indicated by the proportion of large fish and (iii) fishing mortality rates for exploited species that lead to maximum sustainable yield, F MSY, are being proposed to support implementation of the Marine Strategy Framework Directive. Efforts to reconcile any trade-offs among objectives need to be informed by knowledge on the consequences of alternate management actions. 2. We develop, calibrate and apply a multispecies size spectrum model of the North Sea fish community to assess the response of populations and the community to fishing. The model predicts species' size distributions, abundance, productivity and interactions and therefore provides a single framework for evaluating trade-offs between population status, community and food web structure, biodiversity and fisheries yield. 3. We show that the model can replicate realistic fish population and community structure and past responses to fishing. We assess whether meeting management targets for exploited North Sea populations (fishing species at F MSY ) will be sufficient to meet proposed targets for biodiversity and food web indicators under two management scenarios (status quo and F MSY ). 4. The recovery in biodiversity indicators is 60% greater when fishing populations at F MSY than if status quo (2010) fishing rates are maintained. The probability of achieving a food web target was 60% under both scenarios in spite of major community restructuring revealed by other indicators of community size structure. 5. Synthesis and applications. Our model can be applied to evaluate indicator targets and trade-offs among fisheries and conservation objectives. There is a significant probability that reductions in fishing mortality below F MSY would be needed in Europe if managers make a binding commitment to a proposed large fish indicator target, with concomitant reductions in fisheries yield.
ICES Journal of Marine Science, 2009
Piet, G. J., van Hal, R., and Greenstreet, S. P. R. 2009. Modelling the direct impact of bottom trawling on the North Sea fish community to derive estimates of fishing mortality for non-target fish species. – ICES Journal of Marine Science, 66: 1985–1998. This study introduces a spatially explicit model that combines abundance data for all the main fish species in the demersal North Sea fish community with international effort data and estimates of gear-, species-, and size-dependent catch efficiency to determine the mortality of non-target fish species caused by bottom trawl fisheries and its spatial variation. Where necessary information was lacking, assumptions were made, and a sensitivity analysis performed to examine the impact of these issues on model results. Model outcomes were validated using international landings and discard data for five target species: cod, haddock, whiting, sole, and plaice. This showed that depending on its configuration, the model could reproduce rec...
Changes in the North Sea fish community: evidence of indirect effects of fishing?
ICES Journal of Marine Science, 2005
We investigate changes in the North Sea fish community with particular reference to possible indirect effects of fishing, mediated through the ecosystem. In the past, long-term changes in the slope of size spectra of research vessel catches have been related to changes in fishing effort, but such changes may simply reflect the cumulative, direct effects of fishing through selective removal of large individuals. If there is resilience in a fish community towards fishing, we may expect increases in specific components, for instance as a consequence of an associated reduction in predation and/or competition. We show on the basis of three long-term trawl surveys that abundance of small fish (all species) as well as abundance of demersal species with a low maximum length (Lmax) have steadily and significantly increased in absolute numbers over large parts of the North Sea during the last 30 years. Taking average fishing mortality of assessed commercial species as an index of exploitation...
Modelling recovery of Celtic Sea demersal fish community size-structure
Fisheries Research, 2013
The Large Fish Indicator (LFI) is a size-based indicator of fish community state. The indicator describes the proportion by biomass of a fish community represented by fish larger than some size threshold. From an observed peak value of 0.49 in 1990, the Celtic Sea LFI declined until about 2000 and then fluctuated around 0.10 throughout the 2000s. This decline in the LFI reflected a period of diminishing 'large' fish biomass, probably related to high levels of size selective fishing. During the study period, fishing mortality was maintained at consistently high values. Average biomass of 'small' fish fluctuated across the whole time series, showing a weak positive trend in recent years. Inter-annual variation in the LFI was increasingly driven by fluctuation in small fish biomass as large fish biomass declined. Simulations using a size-based ecosystem model suggested that recovery in Celtic Sea fish community size-structure (LFI) could demand at least 20% reductions in fishing pressure and occur on decadal timescales.
A length-structured partial ecosystem model for cod in the North Sea
Fisheries Research, 2010
In this paper we present a general modelling approach for coupled length-structured fish populations. The model includes a target set of fish species which are modelled with full length structure from egg to adult, together with a highly simplified size-spectrum representation of the rest of the ecosystem (zooplankton, benthos and other fish). For the structured fish species the food requirements for growth, maintenance and reproduction, together with a length-based prey preferences, are used to calculate predation mortalities on the prey. Model output can be assessed against a wide variety of independent observations. As an example we develop a model for cod (Gadus morhua) and nine of its most important fish predators and prey in the North Sea. The model is confronted with time series of stock biomass, recruitment, and landings, as well as survey data on length distributions, and diet data. The results suggest that herring (Clupea harengus) predation on early life history stages of cod is dynamically important, and that high herring abundance may play a role in the decline of stocks even during periods of declining fishing pressure. We show that the maximum sustainable yield of cod is strongly dependent on herring abundance, and that current levels of cod exploitation may become unsustainable if herring recruitment returns to historical high levels.
2006
Pope, J. G., Rice, J. C., Daan, N., Jennings, S., and Gislason, H. 2006. Modelling an exploited marine fish community with 15 parameters e results from a simple size-based model. e ICES Journal of Marine Science, 63: 1029e1044.
ICES Journal of Marine Science, 2014
Pelagic fish are key elements in marine foodwebs and thus comprise an important part of overall ecosystem health. We develop a suite of ecological indicators that track pelagic fish community state and evaluate state of specific objectives against Good Environmental Status (GES) criteria. Indicator time-series are calculated for the EU Marine Strategy Framework Directive "Celtic Seas" (CS) and "Greater North Sea" subregions. Precautionary reference points are proposed for each indicator and a simple decision process is then used to aggregate indicators into a GES assessment for each subregion. The pelagic fish communities of both subregions currently appear to be close to GES, but each remains vulnerable. In the CS subregion, fishing mortality is close to the precautionary reference point, although the unknown dynamics of sandeel, sprat, and sardine in the subregion may reduce the robustness of this evaluation. In the North Sea, sandeel stocks have been in poor state until very recently. Pelagic fish community biomass is slightly below the precautionary reference point in both subregions.