Planktivore Management - Linking Food Web Dynamics to Fisheries in the Baltic Sea

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HELSINKI COMMISSION HELCOM HABITAT 10/2008 Nature Protection and Biodiversity Group Tenth Meeting Warsaw, Poland, 5-9 May 2008

Agenda Item 4.4 Viable populations of species

Document code: 4.4/10/INF Date: 29.4.2008 Submitted by: Sweden

PLANKTIVORE MANAGEMENT - LINKING FOOD WEB DYNAMICS TO FISHERIES IN THE BALTIC SEA

The Meeting is invited to take note of the information.

Note by Secretariat: FOR REASONS OF ECONOMY, THE DELEGATES ARE KINDLY REQUESTED TO BRING THEIR OWN COPIES OF THE DOCUMENTS TO THE MEETING

SWEDISH BOARD OF FISHERIES

A new research programme for implementing ecosystem based fisheries management in the Baltic Sea:

Planktivore management - linking food web dynamics to fisheries in the Baltic Sea

Background

Global deterioration of fish stocks and increasing evidence of negative indirect ecosystem effects of fishing, including trophic cascades and major ecosystem changes, emphasise the worldwide need for ecosystem based management of fisheries resources.

The Baltic Sea underwent a dramatic ecosystem shift in the 1980s, from a cod dominated system to a sprat dominated system. The decline in cod, induced by direct overfishing and unfavourable spawning conditions (low salinity), has most likely effected not only the growth and abundance of predators such as cod and zooplanktivores (sprat and herring), but has cascaded through the food chain affecting the whole offshore ecosystem from phytoplankton (more frequent and severe summer blooms) to sea birds. The decline in the cod stock coincided with a marked climate change, strengthening the ecosystem change by an increase in sprat recruitment. It is questionable whether the ecosystem shift would be reversible in the foreseeable future by reducing the fishing pressure on cod or by improved cod spawning conditions, as the ecosystem may have shifted to an alternative stable state, locked in place by the predation on cod eggs by sprat, and the top-down control of sprat on zooplankton food for cod recruits. In case of the latter, a reduction of the sprat stock may be a necessary complementing fisheries management strategy to that of decreasing cod catches, to be able to restore the Baltic Sea ecosystem services.

Parallel with the shift in the offshore ecosystem, the dominating coastal predators perch and pike have declined markedly in the Baltic Proper, most likely due to recruitment failure caused by larval starvation. Research indicates that food limitation of coastal predators may, similar to the situation in the open sea, be caused by the top-down control on the zooplankton community by sprat. Moreover, small-scale experiments in coastal areas of the Baltic Sea suggest that removal of coastal top-predatory fish results in a trophic cascade with increased production of bloom-forming filamentous macroalgae. It is uncertain whether the present state of the coastal ecosystem will persist without influence from open sea interactions, or whether the changes in the coastal ecosystem are linked to the ecosystem shift in the open sea with the top-down control by zooplanktivores (sprat and sticklebacks) on zooplankton. A reduction of zooplanktivorous species may in the latter case be a potential strategy to regain predator domination also in the coastal system.

Within the overall aim of contributing towards a scientific foundation for an ecosystem based management of fisheries resources of the Baltic Sea this project has two main objectives:

SWEDISH BOARD OF FISHERIES

a) To investigate how fish predators can be reinstated to dominate the coastal and offshore ecosystems in the Baltic Sea;

b) To identify the ecosystem effects of changing the abundances of predatory fish in these ecosystems.

Both objectives address the question about the potential for alternative stable states, and whether zooplanktivore reduction may be a potential measure to regain predator dominated coastal and offshore ecosystems. The second objective further addresses the effectiveness of the proposed measures with regard to indirect ecosystem effects of restoring populations of predatory fish, such as, reducing algal blooms and oxygen deficiency. Before any such management measure can be implemented on a Baltic-wide scale, however, its usefulness and potential consequences need to be evaluated scientifically and socio-economically, in ways credible to the international scientific community and management.

Realisation

The work within this project is structured into in five work packages (WPs), broadly based on the different methodologies used to address the main objectives and specific questions.

1) Monitoring and the use of natural gradients will investigate ecological mechanisms structuring the ecosystem, the occurrence of trophic cascades at different spatial and temporal scales and the mechanisms linking the open sea and coastal systems.

2) Field experiment will examine whether the state of the coastal system, primarily the starvation of perch and pike larvae, can be altered by manipulating zooplanktivore or predator density. The WP will investigate what zooplanktivore species may be responsible for the low zooplankton density and where in the coastal-offshore gradient predation on zooplankton takes place with effects on the coast. This WP will also explore the cascading effects of increased predator abundance. The result will be used to validate the food web models developed in WP 4 and to evaluate zooplanktivore manipulation at a larger scale in WP 5.

3) Laboratory experiments will compare the growth, size specific foraging capacity, and metabolic demands of different zooplanktivorous and piscivorous fish species. These parameters are fundamental to determine the minimal resource requirements of the species involved, and are necessary input parameters for food web modelling in WP 4.

4) Modelling will be used to identify and analyse the driving mechanisms and feedback relations that govern the potential for alternative stable community states

SWEDISH BOARD OF FISHERIES

and trophic cascades in the Baltic Sea coastal and offshore ecosystems. The work is fundamental for the application of the project results within ecosystem based management explored in WP 5.

5) Syntheses, strategy evaluation and risk analyses will be performed to contribute towards a scientific platform necessary for an ecosystem-based fisheries management (EBFM) in the Baltic Sea. Project synthesising will be performed using food web models for strategy evaluation and risk analytical modelling, combined with socio-economic analyses to explore different management scenarios.

The project extends over 6 years (2008 – 2013). The work within WPs 1 - 4 are highly interconnected and will overlap in time. The information from all the WPs, in particular WP 4 will subsequently be synthesised in WP 5.

Relevance and potential impact of project

The current research program is a commission from the Swedish Government given to the Swedish Board of Fisheries and the Swedish Environmental Protection Agency:

“The Swedish Board of Fisheries shall, in a local area, conduct an experiment on sprat thinning to evaluate if the method can contribute to create ecological balance in the Baltic sea . During the first phase (2007), a pilot study and planning should be performed to evaluate alternative strategies, organise international collaborations and conduct stakeholder dialogues. The long-term project should be evaluated to produce knowledge regarding the cost efficiency and effects on the environment, as well as the future possibilities to implement the measures on a larger scale. The planning phase shall be reported 31 March 2008, and the final report should be given 31 December 20013”.

The overall purpose of the project is to investigate hypotheses about trophic cascades, including the regulation of predatory fish abundances and ecosystem effects of changes in predator fish stocks. The long-term goal is to contribute to a new knowledge base necessary to convince and help managers and politicians to move towards an ecosystem-based approach to fisheries management in the Baltic sea. Both the Swedish government and the EU aim at a progressive implementation of an ecosystem-based approach to fisheries management.

This project will deliver ecological multi-species models, increased knowledge of the Baltic Sea ecosystem, ecosystem based management decision tools, and consequently contribute to restoration of predatory fish populations in the Baltic sea coastal and offshore ecosystems.

Öregrund 29 April 2008

Magnus Appelberg Mikaela Bergenius Director Project co-ordinator Inst. Coastal Research, Swedish Board of Fisheries