Scottish Marine and Freshwater Science Volume 3 Number 10: Scottish Scallop Stocks - Results of 2011 Assessments

This report presents the results of Scottish regional scallop stock assessments carried out by Marine Scotland Science (MSS) based on commercial catch-at-age and survey data up to and including 2010.

2. Data Collection and Methods

2.1 Assessment Areas

For the purposes of Marine Scotland Science's ( MSS) stock assessments, the scallop grounds around Scotland are divided into assessment areas (previously known as 'Management areas') which are defined on the basis of ICES 2 statistical rectangles ( Figure 2.1.1 and Table 2.1.1). As in previous assessments, rectangle 40E4 is divided into two data components, one from the east side of the Mull of Kintyre and one from the west side. This allows for a clearer distinction between the West of Kintyre and Clyde scallop stocks.

2.2 Fishery Data

The stock assessments use a variety of fishery data which are described further below.

2.2.1 Landings data

The assessments make use of official landings data for both dredge and dive caught scallops. Scottish landings data (landings by UK vessels into Scotland) are collated by Marine Scotland Compliance from sales notes and EU logbooks, and held in the Fisheries Information Network ( FIN) database and in MSS Fisheries Management Database ( FMD).

Landings data for scallops caught in Scottish waters but landed into other (non-Scottish) UK ports were provided by the Marine Scotland Marine Analytic Unit from the i FISH database. These data are only available for the period 2000 to 2010. For the purposes of the assessment, the average ratio of UK (England, Wales and Northern Ireland) landings to UK (Scotland) landings has been used to fill-in the missing historical data for the East Coast, North East and the West of Kintyre, whilst for the Irish Sea, historical data were taken from Howell et al. (2006).

Irish landings data were provided by the Marine Institute, Galway and recent landings into the Isle of Man were provided by Bangor University with permission from the Isle of Man Fisheries Directorate, Department of Environment, Food and Agriculture.

Total landings from each assessment area, by all fishing methods and by all nations, are used in the stock assessments.

2.2.2 Catch-at-age data

Scallop landings are sampled as part of an integrated MSS market sampling programme 3 . Sampling began in the early 1970s, however, it is only since 1982 that sufficient samples have been available to construct reliable catch-at-age data.

Most scallops in Scotland are sold privately, rather than by auction, and are sampled at the processing factories. For each trip sampled, one bag of scallops is selected at random and the lengths of all scallops are recorded to the 0.5 cm below. A sub-sample of the scallops are aged with all individuals age 10 and above recorded in a '10+' age category. Processors handle both dive and dredge caught scallops although dive caught samples are often obtained directly from the dive vessel at the time of landing.

Length at age data and sampled weights for dredge and dive caught scallops are combined and raised to total dredge landings on a quarterly basis. Quarterly data are summed to provide annual catch-at-age (composition) data for Scottish landings. These data are then raised to total annual landings (all nations) to provide input for the stock assessment. Raising factors are determined using a length-total weight relationship applied to the sampled data. Parameters are fixed across stocks and quarters (see Section 2.3).

2.2.3 Discards

Landings (totals and sampled age-composition) are assumed to be representative of catches and no discard sampling takes place. Results of survival experiments (Anon, 1995) suggest that mortality of discarded scallops is relatively low and the assumption of zero discard mortality is therefore unlikely to affect the reliability of the stock assessment.

2.3 Biological Data

2.3.1 Length-weight relationships

Two types of length-weight relationships are used in the stock assessment. A length-total weight (where weight is shell, gonad and muscle weight) relationship is used to raise sampled data to vessel landings. In addition, annual mean meat (muscle) weights at age are derived from the annual age-length composition data and area specific length-muscle weight relationships and these values are then used to convert the stock assessment outputs (which are in terms of numbers) into muscle weights. When insufficient data were available (for example due to missing age classes in particular years), an average of the weight at age over the previous three years was used as a fill-in.

The procedure above differs from the approach taken in previous stock assessments. Output was previously provided in terms of combined muscle and gonad weight which was derived using a quarterly mean weight-at-age fixed over all years (derived from mean quarterly lengths over 1991-1993 and a length-'muscle+gonad' weight relationship).

Neither of the length-weight relationships have been updated in recent years. The stock specific parameter values are given in the relevant stock sections of the report.

2.3.2 Natural mortality

Natural mortality is not precisely known but in common with other fish and shellfish stocks of similar longevity (up to 20 years) it is assumed to be 0.15 yr -1 for all ages and areas (Cook et al., 1990).

2.3.3 Maturity

Scallops first spawn in the autumn of their second year and 100% maturity is therefore assumed for age two onwards.

2.4 Research Vessel Surveys

Dredge surveys of the major scallop grounds around Scotland have been carried out by MSS since the mid 1990s (partial surveys of the west coast began in the late 1980s). There are three surveys a year which, collectively, cover the grounds of the west of Scotland, the North Sea (Scottish coast) and Shetland. The surveys have fixed stations. The station locations were determined with reference to sediment type, using British Geological Survey charts to locate sediments suitable for scallops and knowledge of the scallop fishing grounds contributed by skippers fishing at the time when the surveys first took place. The gear set‑up consists of one array of standard commercial spring-loaded Newhaven type dredges (2.5' wide, 9 tooth bar, with 80 mm internal diameter belly rings, Type A), and another array of smaller configuration sampling dredges with 11 teeth and smaller diameter belly rings similar to commercial gear for queen scallops Aequipecten opercularis (2.5' wide, 11 tooth bar, with 60 mm internal diameter belly rings, Type B).

At each station the dredges are towed at a speed of about 2.5 knots for approximately 30 minutes and all scallops caught are aged and measured (length to the 0.5 cm below). Over the years, different survey dredge widths have been used. Catch rates are, therefore, standardised for both fishing time and dredge width and are presented as numbers caught per hour per metre dredge width (N hr -1 m -1). Indices for each assessment area are calculated by aggregating total catch at age numbers from both dredge types over all hauls and dividing by total duration (and dredge width).

2.5 Assessment

Previous Scottish scallop stock assessments were carried out using a quarterly ( VPA) (Howell et al., 2006). A VPA does not provide an estimate of in the most recent year and additional data or assumptions were required to enable estimation of the stock status in the most recent year. Previously this was achieved by assuming that the temporal trend in fishing mortality was driven by changes in fishing effort ('effort tuning') or by making the assumption that the final year exploitation pattern was equal to the average (with tri-cubic weighting) over the available time series. Due to continuing effort data quality issues and potential inadequacies in the average exploitation pattern assumption, in the assessments presented here the scallop survey data were used as a tuning index within the stock assessment model. In previous assessments, the survey data were used only qualitatively. Results were presented spatially, in terms of total catch rates at each station, and were also used for comparison with the fishery data based stock assessment results (for example comparing recruitment estimates to survey catch rates of individuals < 100 mm).

A number of potential alternative assessment methods were considered, including the Lowestoft VPA suite of programs (Darby and Flatman, 1994). The Time Series Analysis ( TSA) approach was chosen as it is was deemed to have a number of specific advantages over typical VPA type approaches including:

  • Allows fishing mortality estimates to evolve over time in a constrained manner.
  • Provides precision estimates of estimated parameters (numbers at age and fishing mortality at age).
  • Can cope with the omission of catch or survey data if data are of poor quality or missing.
  • Allows survey catchability to evolve over time.

TSA is not a conventional time series model in that it does not include autoregressive or moving average terms. It is a state space model with the state of the stock in a particular year described by a vector of stock numbers at age and fishing mortality at age (the 'state vector'). The 'state equations' define how this vector changes over time i.e. how the numbers at age in a particular year relate to the numbers at age and fishing mortality at age in the previous year. This vector is related to the data or observations (typically catch‑at-age data and survey data) through 'observation equations'. The filter is used to estimate the state variables. The method was derived by Gudmundsson (1994) and further developed by Fryer (2002) for use in the assessment of North Sea and West of Scotland demersal fish stocks ( ICES, 2011).

The model is initialised and run through a series of scripts although actual parameter estimation is carried out by a programme which is automatically called from within R.


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