Perceptions of the North Sea Whiting Stock

Description

This project was undertaken under the Scottish Industry Science Partnership (SISP) in partnership with the Shetland Fishermen’s Association, the Orkney Fisheries Association and the Scottish Fishermen’s Federation, and with the assistance of the Marine Scotland Science Marine Laboratory in Aberdeen.

The principal aim of the project was to investigate differences between the scientific and industry perceptions of the whiting stock in the northern North Sea, and in particular any differences in the catchability of whiting at age between scientific and commercial fishing gears, and between different areas.

Start Date and Duration

The project ran from November 2010 until the end of 2011.

Funding

The project was funded through the Scottish Industry Science Partnership (SISP).

Summary

This project was motivated by the apparent differences that existed in 2008/09 between fishermen’s and scientists’ perceptions of the state of the whiting stock in the waters around Orkney and Shetland. At that time the perception amongst fishermen was of a high, and increasing, abundance of whiting which they did not believe had been detected by scientific assessments. (Subsequent assessments revised the estimated size of the whiting stock upwards by a substantial amount.)

To explore possible reasons for this difference in perceptions this project collated and compared large quantities of data from a variety of sources to explore a number of issues, including:

  • The Distribution of Commercial Fishing Activity for Whiting
  • Distribution of Whiting in Relation to Depth
  • Distribution of Whiting in Relation to Sediment Type
  • Distribution of International Bottom Trawl Survey Hauls
  • The Definition of ‘Soft’ Fishing Grounds
  • Fishing Vessel - Research Vessel Calibration
  • Survey Catchability and Stock Assessment Biases
  • Regional Trends in Stock Assessment

Key Points

The following section very briefly summarises the principal findings of this project.

  • Evidence of northwards and westwards shift in distribution of whiting from 2006/07 to 2009/10.
  • Overall 8% increase in whiting catch rates over this period.
  • Some evidence of relationship between the distribution of whiting and water depth, but not straightforward.
  • Commercial catch rate of whiting highest on less gravelly sediments. In general, catch rate increased as gravel content of sediment decreased, and as mud content increased (except for most muddy sediments).
  • Catches by fisheries research vessels showed opposite trend: higher whiting catch rates on more gravelly and less muddy sediments.
  • Commercial fishing parameters (effort, whiting landings, catch rate) in areas where International Bottom Trawl Survey (IBTS) hauls were made similar to overall averages. (But a substantial proportion of IBTS hauls made in areas with no commercial effort.)
  • No relationship between catch rates of whiting by survey vessels and commercial vessels in same areas.
  • IBTS hauls more likely to be on sediments with low gravel and mud contents. Evidence of changes over time in proportions of hauls on different sediment types (less on least gravelly sediments, more on more gravelly sediments).
  • Distribution of IBTS hauls in relation to sediment type differed markedly to that of commercial fishing effort.
  • Evidence that changes in distribution of IBTS hauls in relation to sediment type, and differences from distribution of commercial effort, could affect whiting catch rates calculated from IBTS data. (Not known how this might affect whiting stock assessments.)
  • ‘Soft’ fishing grounds primarily those sufficiently smooth and free from obstructions to be fished with seine nets or light trawls. Not necessarily composed of soft sediments.
  • Known ‘soft’ grounds around Shetland accounted for higher than average levels of fishing effort and whiting landings, but had lower than average whiting catch (landing) rates.
  • Planned intercalibration between commercial fishing vessel and fisheries research vessel curtailed by bad weather, but no evidence of gross differences between vessels’ catches. Was evidence of differences in size range of whiting caught by fishing and research vessels.
  • Evidence that assumptions in current assessment model about catchability at age result tend to result in lower biomass and higher fishing mortality estimates, but not clear that changing these assumptions would result in a ‘better’ or more optimistic assessment or in significantly different catching opportunities.
  • Low catchability of older whiting in scientific surveys does not, in itself, appear to represent a source of bias in the assessment, but the way in which catchability is modelled could introduce biases.
  • Recommended that statistical catch at age assessment models, with flexible and statistically rigorous modelling of catchability and sources of uncertainty, be explored for use in future North Sea whiting assessments.
  • Evidence of some regional trends in abundance of North Sea whiting, but not strongly reflected in patterns of recruitment or maintained throughout the year. Appear to reflect changes over time in the movement of whiting (especially between north and south) rather the existence of separate stocks.
  • Not possible, on the basis of this analysis, to recommend separate assessments for whiting in different areas, or to provide guidance on the definition of regional stock units.

Download the Project Report

Comparison of Industry and Scientific Perceptions of the North Sea Whiting Stock (pdf)