Working Group with the Aim to Develop Assessment Models and Establish Biological Reference Points for Sea Trout (Anadromous Salmo trutta) Populations (WGTRUTTA; outputs from 2019 meeting)

The Working Group WGTRUTTA was established in 2017 with the Aim to Develop Assessment Models and Establish Biological Reference Points for Sea Trout (Anadromous Salmo trutta) Populations. The WG has representatives from every country containing a self-reproducing population of sea trout throughout Europe, in total 19 countries.

Four subgroups worked to deliver the three ToR: 1) compile information from a selection of suitable rivers across Europe with long-term data on parameters such as juvenile densities, habitat characteristics and, if available, abundances of ascending spawners and out-migrating smolts; 2)develop new, validate and fine tune existing population models for sea trout; 3) establish  and  evaluate different approaches for estimating Biological Reference  Points  (BRPs)  across  regions  with different characteristics and conditions for sea trout.

The sea trout database structure was completed and populating it with data is well underway. This database is designed to provide a central depository for data used by the WG, and consists of two components: for environmental and bio-ecological data. The WG has created an inventory of data collection methods across the 19 countries of the natural range. There are common methodological  approaches  but  few,  if  any,  that  are  uniform  across  all  countries.  An  inventory  of  Passive  Integrated  Transponder  (PIT)  tagging  infrastructure has also been created and will be made available via a mapping tool. The WG are liaising with ICES and their Regional Database and Estimation System (RDBES), working towards a time when ICES will host the WGTRUTTA database.

The WG undertook a comprehensive review of the scientific literature on ecological factors affecting the abundance and life history of anadromous fish, which has been published in Fish and Fisheries (Nevoux et al. 2019). This provides the knowledge base to support development of pop-ulation models, taking into account these complexities in the life history of the resident and anad-romous components of stocks.

The WG has developed a set of length-based indicators to assess the status of a stock (after the Workshop on the Development of Quantitative Assessment Methodologies based on LIFE-history  traits,  exploitation  characteristics,  and  other  relevant  parameters  for  data-limited  stocks (WKLIFE)), using index catchments to demonstrate these indicators and to identify where pres-sures may have had an impact. Two papers, both published, have been developed describing the development and application of these length-based indicators of sea trout stock status (Shephard 2018a, Shephard, 2019).

The WG has extended the development and application of the Trout Habitat Scores (THS) model using Baltic data from Sweden, and commenced testing this with data from Northern Ireland. A theoretical Bayesian Population Dynamics Model for Baltic Sea trout is also being developed.

The challenges of developing and applying a BRP approach to sea trout were further explored by applying several curve fitting approaches (including Beverton-Holt, Ricker, Hockey Stick) to ‘data rich’ stocks with data from counts, returning stock estimates, catches, and juvenile abundance surveys. A ‘one-size-fits-all’ option is highly unlikely, but a suite of tools is more promising, especially if they can be targeted towards a relatively small number of sea trout stock group-ings. A grouping proposed for 16 sea trout stocks in England and Wales, based on growth rates and longevity, has been identified as a potential stock grouping tool and it is proposed to test and develop this across the natural range of the species in future research. Such groupings might be used as the basis for focussing stock-recruitment or other model approaches, and/or to make recommendations on selecting index rivers and data collection programs.