An overview of the estimates of marine survival for wild and hatchery-reared smolts returning to homewaters (i.e.
before homewater exploitation) for the 2001 and 2000 smolt year classes (returning 1SW and 2SW salmon, respectively) is presented in Figure 3.3.1.1. The survival values presented are standardized (Z-score) indices relative to the averages of the time series. The original survival indices for different rivers and experimental facilities are presented in Tables 3.3.1.1 and 3.3.1.2.
With the exception of the Northern NEAC hatchery indices, Northern and Southern NEAC areas show a general decline in marine survival over the past 10-20 years (Figure 3.3.1.1). The steepest decline appears to be for the wild smolts in the Southern NEAC area.
In general, a majority of the survival indices for the latest smolt year classes for both wild and hatchery-reared smolts were below those of the previous year and the 5- and 10-year averages (Tables 3.3.1.1 & 3.3.1.2). Return rates of hatchery released fish, however, may not always be a reliable indicator of marine survival of wild fish.
Results from these analyses are consistent with the information on estimated returns and spawners as derived from the PFA model (section 3.3.4), and suggest that returns are strongly influenced by factors in the marine environment.
3.3.2 The NEAC – PFA model Description of model
The Working Group has previously developed a model to estimate the pre-fishery abundance (PFA) of salmon from countries in the NEAC area. PFA in the NEAC area is defined as the number of 1SW recruits on January 1st in the first sea winter. The method employs a basic run-reconstruction approach similar to that described by Rago et al. (1993) and Potter and Dunkley (1993). The model estimates the PFA from the catch in numbers of 1SW and MSW salmon in each country. These are raised to take account of minimum and maximum estimates of non-reported catches and exploitation rates of these two sea-age groups. Finally these values are raised to take account of the natural mortality between January 1st in the first sea winter and the mid-point of the respective national fisheries. As reported last year (ICES 2002/ACFM:14), the Working Group has determined an ‘m’ value of 0.03 per month to be appropriate. A Monte Carlo simulation (1000 runs) using ‘Crystal Ball’ in Excel (Decisioneering, 1996) is used to estimate confidence limits on the PFA values. Potter et al. (1998) provides full details of the model.
3.3.3 Sensitivity analysis of the PFA model
A sensitivity analysis for the spreadsheet model which generates PFA estimates in the NEAC area was described in ICES 2002/ACFM:14.
The sensitivity of the overall assessment of PFA for the NEAC Area, and for the Northern and Southern European stock complexes, depends on the values of the various parameters provided for different countries, and these will also be weighted by the national catches. The analysis provided an evaluation of the effects (% change) on the assessment of PFA of maturing and maturing 1SW salmon from Northern and Southern Europe of making changes to the non-reporting rate (‘R’), the exploitation rate (‘U’) and the time of return to homewaters (‘t’).
Country (Region) Sea-age Parameter
Norway (mid) 1SW Non-reporting rate
Norway (North) MSW Non-reporting rate
Ireland 1SW Non-reporting rate
Ireland 1SW Exploitation rate
Scotland (East) 1SW Exploitation rate
Scotland (East & West) MSW Exploitation rate
Scotland (East) MSW Non-reporting rate
No changes were made to any of these variables prior to running the NEAC PFA model for the 2003 assessment.
3.3.4 Grouping of national stocks
National outputs of the NEAC PFA model are combined in the following groups to provide NASCO with catch advice or alternative management advice for the distant water fisheries at West Greenland
and Faroes.
Southern European countries: Northern European countries:
Ireland Finland
France Norway
UK(England & Wales) Russia
UK(Northern Ireland) Sweden
UK(Scotland) Iceland
The groups were deemed appropriate by the Working Group as they fulfilled an agreed set of criteria for defining stock groups for the provision of management advice that were considered in detail at the 2002 meeting (ICES 2002/ACFM:14). Consideration of the level of exploitation of national stocks at both the distant water fisheries resulted in the proposal that that advice for the Faroes fishery (both 1SW and MSW) should be based upon all NEAC area stocks, but that advice for the West Greenland fishery should be based upon Southern European MSW salmon stocks only (comprising UK, Ireland, and France).
3.3.5 National input to the NEAC PFA model
To run the NEAC PFA model most countries are required to input the following time-series information (beginning in 1971) for 1SW and MSW salmon:
• Catch in numbers
• Unreported catch levels (min and max)
• Exploitation levels (min and max)
In some instances, the above information has been supplied in two or more regional blocks per country. In these instances, the model output is combined to provide one set of output variables per country. Descriptions of how the model input has been derived were presented in detail at the Working Group meeting in 2002 (ICES 2002/ACFM:14).
Where there have been modifications to these derivation methods an explanation is given below. The input values for the required variables are provided in Table 3.3.3.1a-u.
Changes were made to the exploitation and unreported inputs for the Swedish data based on re-consideration of
The National Conservation limits model has been designed as a means to provide a preliminary Slim reference point for countries where river-specific reference points have not been developed. These figures should also be regarded as uncertain and should only be used with caution in developing management options. A drawback with an overall national status of stocks analysis is that it does not capture variations in status in different fishery areas or stock complexes; something that has been addressed, at least in part, by the area splits in some countries.
The model output for each country has been displayed as a summary sheet (Figures 3.3.4.1(a to j)) comprising the following:
• Estimated total returns and spawners (±SD)(derived from the National Conservation Limit model).
• Estimated total catch (including non-reported) of 1SW and MSW salmon.
• Estimated pre-fishery abundance (PFA) of maturing 1SW and non-maturing 1SW salmon (labelled as 1SW and MSW).
• Total exploitation rate of 1SW and MSW salmon estimated from the total returns and total catches derived from the model.
• National stock-recruitment relationship (PFA against lagged egg deposition), with Slim fitted by the method presented in ICES 2001/ACCESS:14.
A brief description is given below summarising the outputs from the model.
Finland: Finnish salmon essentially comprise a single river stock, the River Teno (Tana). The data inputs include both Finnish and Norwegian catches for this river. The assessment suggests that the numbers of returns and spawners have fluctuated widely since 1971. The early part of the time-series (1971 to 1975) is characterised by a steep rise, followed by a sharp decline. Numbers of returns and spawners remained low until 1982, but have shown a steady increase since this time, reaching a peak in 2000. In the last two years both returns and spawners have shown a steep decline.
France: Returns and spawners are estimated to have declined over the past 20 years, although there have been large annual fluctuations. Numbers have been particularly low in recent years, with the last eight years being the lowest in the time-series. There has also been a decline in the proportion of MSW salmon in the catch over the time-series. The current status of the stocks must therefore be considered to be low with no indication of a recovery.
Iceland: The assessment suggests that there has been an overall decline in total returns of salmon to Iceland, from around 120,000 in the 1970s to about 60,000 in 2002. However the 2002 values for both returns and spawners are greater than observed in the two previous years. Estimated returns showed an upward trend in the early part of the time-series (1971-78), followed by a sharp decline (1979-84) and a brief recovery to early levels in the late 1980s. There has been a clear downward trend since 1988. There has also been a marked decline in MSW salmon relative to 1SW fish.
Ireland: Estimates of PFA and spawning stocks for Ireland show significant fluctuations over time and three distinct periods are indicated with highest abundance in the 1970’s, lower abundance in the 1980’s, and the lowest abundance occurring in the 1990’s. The early part of the time-series (1971 to 1981) is characterised by a steep rise to the maximum value in the entire time-series, followed by a sharp and prolonged decline. A subsequent recovery period is noted from 1981 to 1989, although the values did not rise to the levels observed in the earlier part of the time-series. A period of steep decline occurred over the period 1989 to 1992 with stock levels fluctuating around a new, lower, level for the remainder of the time series. The status of the stocks must therefore be considered to be low with no significant recovery in the last decade.
Norway: Before 1983 the catch data were considered to be unreliable. Therefore, only catch information after this date were used for the development of the national PFA estimates. The data for the Norwegian part of the River Tana (Teno) are included in the Finnish PFA estimates. There was a decline until the late 1990s thereafter, a sustained increase in returns was observed over the period 1998-2001 but a decline was observed in 2002. The spawning stock has remained relatively constant throughout the period due to a reduced exploitation rate in the second half of the time period.
Russia: Total returns to Russia are estimated to have been generally greater in the early part of the time series. From 1987 onwards there has been a slight upward trend in the number of returns although the estimates in the last two years counter this general trend. Estimates of spawners follow a similar pattern to that described for returns. There has been a
Sweden: Stocks in Sweden have fluctuated widely throughout the time-series and following a substantial decline in the mid-1990s, there has been a rapid recovery followed by successive and moderate declines in the last two years. A feature of the latter half of the time-series is the increasing proportion of the stock that is comprised of MSW salmon.
The exploitation rate has remained high over the last 30 years although there has been a decline from 1990 onwards.
UK (England & Wales): Stocks are estimated to have declined over the past 30 years, although there have been large annual fluctuations. The estimated PFA has declined more rapidly for MSW than 1SW salmon. There has been a slight up-turn in overall PFA since 1997, the lowest in the time-series. The decline in spawner numbers is less marked than that for the returns, reflecting a reduction in the homewater exploitation rate in the last decade.
UK (Northern Ireland): Stocks are estimated to have declined slowly during the 1970’s and early 1980’s, increasing again in the 1990’s. However, estimates of PFA2 for the last four years being among the lowest over the period. The catch is dominated by 1SW fish, but there are uncertainties in the relative status of 1SW and MSW fish, as the data on catch composition by sea age are uncertain for most of the historical time-series.
UK (Scotland): The assessment indicates that stocks have fallen markedly since the early 1970s, although the decline in total spawner numbers has been less marked than those of homewater returns, reflecting the reduction in homewater exploitation rates. The estimated return rates for the last seven years are the lowest in the time series.
3.3.7 Summary of status of stocks
The marine survivals of wild and hatchery-reared smolts in both Northern and Southern NEAC areas show an overall decline over the past 20 years. The steepest decline is that in the wild smolts in Southern NEAC area (Figure 3.3.1.1).
Survival of both wild and hatchery fish in the Northern NEAC area, however, have generally increased since 1997.
In general, the total returns of salmon and spawning stocks in the Northern NEAC area, as derived from the NEAC PFA model, have fluctuated for past 30 years but have undergone a relative increase since 1998, followed by a new decrease in the very last years for 1SW salmon. In contrast, salmon stocks in Iceland show a slow but constant decline since 1985 for both 1SW and MSW salmon.
Salmon stocks in the Southern NEAC area show a consistent declining trend over the past 30 years. This relates especially to the MSW component of the salmon stocks.
The consistent trends in marine survival of smolts and the estimated returns and spawners as derived from the PFA model underline the effect of factors in the marine environment on the number of returns.
3.4 Development of Age-Specific Conservation Limits