Electronic tagging of marine fish to determine movement patterns
Dr D. Sims (MBA Fellow), Dr J. Metcalfe, Dr D. Righton (CEFAS Laboratory, Lowestoft)
Recording the individual movements and behaviour of marine fish by fitting them with electronic tags is fast becoming a central method for understanding the dynamics of distribution changes, response to environmental heterogeneity, and spatial structuring of natural populations. During the past year, long-term tagging studies between MBA and CEFAS funded by Defra and other agencies have continued with ongoing research on migrations and habitat selection of basking shark (Cetorhinus maximus), rays (Raja spp.) and Atlantic cod (Gadus morhua). New advanced technology such as pop-up archival transmitting tags, mobile telephone tags and data storage tags, have all been used to record, store and retrieve long-term behavioural data. These large data sets are being analysed to test numerous hypotheses in the field of behavioural ecology. A principal aim is to understand how fish view their environment and respond to changes, and what habitats they select over others and why. Understanding when and why fish behave the way they do has involved developing computer simulations to test against observed behaviours to reveal the underlying decision-making processes. A key focus here has been to identify whether common behavioural “rules” exist across diverse species, from sharks to cod. Knowledge of the behavioural “rules” underpinning movements such as dispersal and migration will enable predictive models of movement to be developed. This should provide valuable data for parameterising the new generation of spatially-structured fish population models that will be crucial to an ecosystem-based approach to fisheries management.
The MBA – CEFAS collaboration in this area has also spawned a new European project in the Census of Marine Life. “European Tracking of Predators in the Atlantic – EUTOPIA” involves scientists in the field of marine vertebrate telemetry throughout the UK and Europe. The long-term aim is to determine how marine predators utilise key habitats in the ocean, such as fronts, and how movements, and consequently spatial abundance and distribution patterns may alter in response to environmental change.
Development of a dynamic ‘prey seascape’ for analysing foraging behaviour
Dr D. Sims (MBA Fellow), Dr A. Richardson (SAHFOS, University of Queensland & CSIRO Marine Research, Australia), Mr M. Witt (University of Exeter), Dr J. Metcalfe (CEFAS, Lowestoft)
Movements of wide-ranging top predators can now be studied effectively using satellite and archival telemetry. However, the motivations underlying movements remain difficult to determine because trajectories are seldom related to key biological gradients, such as changing prey distributions. A major limitation to progress is the lack of prey fields in the open ocean. Over the past year we have developed a dynamic “prey seascape” of zooplankton biomass in the north-east Atlantic Ocean to examine active habitat selection in the plankton-feeding basking shark Cetorhinus maximus. The prey field was constructed from zooplankton samples collected by the Continuous Plankton Recorder (CPR) survey. It is a near-surface, prey-biomass field with minimum spatial and temporal resolutions of 56 x 36 km and 14 days respectively, and extends to the scale of the north-east Atlantic (2800 x 2200 km), so is well suited to analyse short and long-range movements of basking sharks.
The relative success of shark searches across the prey field was examined by comparing prey biomass encountered by sharks with encounters by random-walk simulations of ‘model’ sharks. Initial results show movements by adult and sub-adult sharks yielded high prey encounter rates than the majority of random-walk simulations. This indicates behavioural patterns were consistent with basking sharks using search tactics structured across multiple scales to exploit the richest prey areas available in preferred habitats. The study highlights how dynamic prey landscapes enable active habitat selection in large predators to be investigated from a trophic perspective, an approach that may inform conservation by identifying critical habitat of vulnerable species.
Interpreting complex fish behaviour patterns using signal processing
Miss E. Shepard (MBA), Dr D. Sims (MBA Fellow), Dr M. Zaki (University of Plymouth)
Patterns of vertical movement in marine fish and other predators can provide important insights into behavioural strategies and decision making, but can also be highly complex, appearing erratic and changing rapidly in response to a changing environment. Specific behavioural bouts are therefore difficult to isolate and identify, especially from the large data sets recorded by electronic tags (once per min for up to a year in some cases). Studies at the MBA have progressed by applying a novel suite of signal processing techniques to fine-scale vertical movements of basking sharks (Cetorhinus maximus) to identify periodicity in diving behaviour. The main periodicities identified were associated with foraging behaviour. Diel vertical migration, a well-documented behaviour in zooplankton, represented the main periodic dive behaviour in basking sharks, occurring for up to 97% of the time in any given month, confirming the importance of plankton movements in determining the vertical habitat selection of these planktivorous sharks. A previously unknown tidal pattern of vertical movement was also identified. The only juvenile tagged showed markedly different patterns of vertical movement, which may be a result of foraging inexperience. Sharks also make dramatic transitions in vertical behaviour at an hourly timescale associated with changes in thermal habitat, indicating that they can respond rapidly to changes in zooplankton availability associated with individual water masses. This research demonstrates the value of using signal processing techniques to help understand the role of environment on fish behaviour, work which we are now expanding to investigate patterns in commercially important fish such as Atlantic cod (Gadus morhua) and bigeye tuna (Thunnus obesus).
Foraging strategy of free-ranging marine fish predators
Dr. N. Hutchinson (MBA), Dr D. Sims (MBA Fellow), Dr. D. Morritt (Royal Holloway, University of London), Dr Richard Thompson (University of Plymouth)
The relative abundance and availability of prey is one of several fundamental aspects governing the distribution of fish predators. Theoretical predictions from the optimal diet model suggest that such predators spend less time per prey item searching for food in productive habitats and hence have a narrower diet than in less productive habitats. Although there have been numerous studies, this idea has not been rigorously tested in the field for predators feeding on mobile prey.
Following the award of a NERC grant, research on this theme has progressed over the last two years by tracking the fine-scale movements of dogfish, a generalist marine fish predator, in relation to prey density gradients. Using a sophisticated radio-acoustic positioning array foraging dogfish were tracked in real time during nocturnal excursions. Invertebrate prey abundance data were collected simultaneously with surveys utilising baited traps and underwater-camera observations. This approach, of tracking predator and prey, was coupled with prey consumption analysis following recovery of stomach contents of dogfish. The capture of females during and post-foraging enabled stomach contents to be sampled non-lethally. Analysis is continuing to determine how foraging behaviour of these mobile predators varies with prey availability across microhabitats varying in quality. This work will help understanding of the behavioural mechanisms of habitat selection in wild fish.
Fish habitat selection: mechanisms underlying differential distribution of the sexes
Miss V. Wearmouth (PhD student, MBA & University of Bristol), Dr. D. Sims (MBA Fellow), Dr. J. Partridge, Prof. I. Cuthill (University of Bristol)
Identifying how fish species differentially distribute themselves in nature is important for the successful management of fish populations. Within-species segregation of the sexes is widespread in the animal kingdom, has dramatic effects on population distribution, yet the underlying causes remain poorly understood. The majority of studies have focused on terrestrial ungulate species, which typically exhibit pronounced sexual dimorphism in body size and, as such, may not be the most appropriate models. This project uses the sexually segregating dogfish, Scyliorhinus canicula, which is monomorphic with respect to body size, as a model species to test hypotheses about the mechanisms of sexual segregation.
The behaviour of wild fish was monitored in a semi-enclosed sea lough using acoustic and archival tracking of individuals over a range of spatio-temporal scales. This revealed sex differences in both vertical and horizontal space use. Female dogfish spend daylight hours refuging in shallow water caves whilst males are found in deeper water, and females generally travel further and target deeper areas than males during nocturnal foraging excursions. Hypotheses informed by observations of wild behaviour have been examined in the laboratory. Using computer-aided video tracking we have investigated whether energetic, predation or social factor hypotheses explain sexual segregation in this species.
It appears the social factor is key: female dogfish refuge to avoid males. Mating in this species is aggressive and levels of male harassment are high, with males targeting refuges in search of opportunistic matings at the times of the day when they are most likely to encounter females leaving or returning from foraging. In addition, laboratory experiments suggest that females select sub-optimal habitats when males are present.This research will improve our understanding of the processes governing sex-based differences in the behaviour and habitat selection of marine fish. It will contribute to knowledge about how natural fish populations are structured and what factors influence changes in spatial distribution through time.
Critical habitat and population size of basking sharks in the north-east Atlantic
Dr E. Southall (MBA), Dr D. Sims (MBA Fellow)
The basking shark (Cetorhinus maximus) is the world’s second largest fish and has been exploited by humans for at least two hundred years. There is current concern that past fisheries have reduced populations to low levels. As such, this species is listed as vulnerable on the IUCN Red List of Threatened Species, and recently included on Appendix II of CITES. Their inherent vulnerability is largely due to k-selected life history traits such as their relatively late age of maturity and low fecundity leading to low potential rates of population growth. Despite this vulnerability, the protection for basking shark in Europe is limited and varies spatially.
To address these concerns, a project funded by the Esmée Fairbairn Foundation was initiated in 2003 to collate and combine behavioural, survey and sightings data collected by a consortium of six UK charities into a single, unique database. Analysis of the database has identified critical habitat of this species on the continental shelf. Furthermore, analysis of space use showed basking sharks move between different economic zones and were not afforded statutory protection for the major part of the time they spent within preferred habitat on the European shelf. This demonstrates the limited capacity of the British protection zone in territorial waters for encompassing the greater part of shark space utilisation. As basking sharks regularly crossed national zone boundaries our project partners, the Marine Conservation Society formally proposed basking sharks be considered for inclusion in the Convention on the Conservation of Migratory Species of Wild Animals. Mostly due to the research undertaken at the MBA, the basking shark was accepted on Appendix I and II of the Bonn Convention on Migratory Species (CMS) in November 2005. This Convention states all nations with basking sharks in their waters must introduce strict legislation to prevent capture and landing of the species.
Molecular population genetics and ecology of sharks
Dr L. Noble, Dr C. Jones (University of Aberdeen), Dr D. Sims (MBA Fellow), Dr J. Metcalfe (CEFAS, Lowestoft)
Recent studies worldwide show coastal and pelagic shark populations are declining due to human exploitation. The ability to identify population effects in specific areas however is hampered by a lack of knowledge about the limits of dispersal, levels of mixing between different geographic regions, and, hence, the presence and nature of sub-population structure. Through Defra funding molecular genetic markers to address these knowledge gaps are being developed for the basking shark, and other threatened species such as the great white (Carcharodon carcharias), porbeagle (Lamna nasus) and mako shark (Isurus oxyrinchus). Progress is also being made with genetic methods to identify shark body parts, which should allow separated fins and other parts to be identified to species level.
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