Address: Our Ref:
Your Ref:
Mr Mike Ashwin
By e-mail Date: 30.11.20
Dear Mike,
River salmon stock estimates Re Cross Border & National Byelaw measures implementation
Thank you for your letter of the 23rd July on your subject above. Firstly, may I apologise for not having responded to that before now – this appears to have been an administrative error that only surfaced when I reviewed your email and attachments of 21st November.
This response will address the issues you raised in your letter of the 23rd July. It includes observations relevant to NRW, and not those which fall under the remit of the Environment Agency – such as questions about the Severn NLO and byelaws.
Firstly, paragraph 6 of your opening page appears to imply that the Local Inquiry in 2019 was in some way flawed in its failure to fully consider the pending review of stock assessment procedures. However, the pending review was referred to in evidence submitted to the Inquiry by NRW and others (including in your own evidence), so the Inspector was aware of it in reporting his findings.
Regarding the review of stock assessment procedures outlined in Action F2 of the NASCO IP 2019-2024: our previous correspondence of the 12th July 2020 addressed this area in some detail. To reiterate, NGOs participating in the Wales Fisheries Forum and the England Fisheries Group will have the opportunity to comment on proposals arising from that review. These NGOs include, for example, the Angling Trust.
For the latter part of your letter headed: Dee index RSE programme…etc.: in part, this re-visits issues raised and addressed in previous correspondence, so I won’t seek to repeat these. NRW does not disagree that declining salmon runs and catches (and the complex of factors potentially influencing the relationship between the two) will, if this trend continues, make estimation procedures which rely on catch data increasingly uncertain. This point was made in our previous correspondence, as was our aim to
seek to strengthen catch-independent methods to assess stocks alongside more established procedures. My colleague Ian Davidson has been in correspondence with Emeritus Professor Brian Revell regarding analyses by Professor Revell pertaining to some of these matters which he’s kindly shared with us (although we are not familiar with the quotations from Professor Revell referred to in your text, which presumably originate from other correspondence?).
Declining stocks and catches also effect the degree of uncertainty around mark- recapture estimates obtained for the Dee. This is largely unavoidable, given that, as fewer salmon return to the river, there are fewer to tag at Chester and fewer fish available to be caught by anglers. Because of this, estimates become less precise as fewer tags are recovered in absolute terms. To address this, we are looking at the possibility of reinforcing the established mark-recapture programme with deployment of automated tag screening facilities and counters associated with fish passage structures – including structures constructed in the coming years as part of the Dee LIFE project.
The use of angling exploitation rate estimates from the Dee in deriving run figures for non-counted rivers in Wales is as described in the FOI response to John Eardley of December 2018 (I presume this is the FOI you are referring to?). The regression models ‘1’ and ‘2’ you refer to are based on exploitation rate data obtained from a number of counted rivers, not just the Dee. Future model developments will also draw on data from all validated counted rivers in England and Wales. Relative differences between angling exploitation rate data obtained for 1SW and MSW salmon on the Dee are, as you indicate, used to adjust exploitation rate estimates derived for all fish to produce sea age specific values. This ‘default’ adjustment is currently applied across E&W.
Regarding your concerns about Dee salmon run estimates based on catches and tags returned by logbook holders, as opposed to the wider angling population: logbook anglers don’t ‘catch 2/3/4 times the proportion of tagged fish in their returns’ as you state. From the mid-1990s to date, the proportion of tagged fish in the reported catch has averaged 18.6% in logbook returns and 16.8% in the returns of all anglers. Those proportions have varied over time, but for the wider angling population, tag reporting rates are not accurately known – hence our focus on logbook returns.
Trap efficiency for salmon (and sea trout) at Chester has been declining gradually since the programme began in the early 1990s. Structurally, the trap has changed little since that time, however, the hours fished per year have reduced – from a 5-year average of ~5,000 hours in the early 1990s to ~4,200 hours in recent times. Trapping continues to be carried out year-round (January – December). These small changes
should not significantly affect the provision of run estimates or other key outputs from the Dee programme.
Salmon run estimates at Chester obtained from the start of the year to mid-October are based on tagging and angling recaptures. Numbers of fish entering after the end of the angling season are estimated from trap catches and average trap efficiencies. Out-of- season trap catches at Chester represent ~10% or less of the annual total.
Where tag recaptures allow, part-season run estimates have been obtained based on blocks of months, but not all individual months. In 2018, for example, the earliest period for which a part-season run estimate was obtained was January – June (684 fish). This compared to an annual run estimate of 3,796 salmon for that year. Annual rod catches and run estimates are strongly correlated.
The sea age, size and sex composition of the run at Chester (along with fishing and natural mortality estimates) are used to evaluate the proportion of the Dee run which survives to spawn and their likely egg contribution.
Monthly age-weight keys for the Dee (and more recently the Tamar) have been widely used on rivers without counters to assign rod caught fish into 1SW and MSW sea age groups as part of the general process for estimating annual egg deposition levels.
I hope this addresses the specific points you raised in July. We will aim to respond to your email communication of 21st November later this week.
Yours sincerely