Variation in the yearly and seasonal abundance of juvenile Atlantic salmon in a long-term monitoring programme

Abstract

The long-term monitoring programme for the River Teno Atlantic salmon (Salmo salar L.) stocks has covered the juvenile densities (25 yr) and the abundance and characteristics of the returning adults (31 yr). The feasibility of the programme was examined by studying the interrelationships between the yearly catches and juvenile salmon densities, performance and reliability of the electrofishing method, and the effects of fishing regulations on the salmon stocks. Finally, juvenile salmon abundances were related to the available fluvial habitat and reference levels were defined by using habitat models.

Extensive seasonal variation in juvenile salmon density was apparent. The densities of fry and parr showed an increase from early summer towards late August and a subsequent decline towards the autumn. Long-term electrofishing monitoring is recommended to be carried out in as standardized a form as possible in order to reduce variations in catchability.

Over the 25-year monitoring period, the abundance of parr (1+) increased in one sampling site cluster out of nine clusters and declined in one cluster. Fry densities increased in seven clusters. Juvenile densities exhibited considerable temporal and spatial variation. Similarly, the salmon catches varied extensively, and the numbers of 1-2SW salmon and previous spawners increased.

The numbers of 1–2SW female salmon in the catches and the subsequent juvenile densities were significantly related, as regression models explained 19–44% of the variation in juvenile abundance. The juvenile monitoring allows evaluation of the relative spawner abundance in preceding years, confirming the information provided by catch statistics.

Juvenile salmon densities explained 23–41% of the variation in subsequent 1–2SW salmon catches. Significant correlations were detected with a lag of one year between the subsequent sea-age groups of salmon in the catches. Thus, these relationships can be used for forecasting future salmon abundances.

Large areas of high habitat quality in the River Teno system fail to meet their expected juvenile densities, and factors others than physical habitat characteristics, such as a lack of spawners, restrict the juvenile abundance. More than 50% of the permanent sampling sites where habitat would predict high densities (≥ 50 parr per 100 m²) had observed densities in the mid (10–49) or low density category (< 10).

It was expected that the densities should increase after regulatory measures implemented in 1989–1990, but results indicate that the reference levels of parr densities have not been attained and the densities have not increased, whereas a general increase in salmon fry densities was detected. Nonetheless, the management measures have succeeded in maintaining the River Teno salmon stocks, which still today enable and support diversified fisheries.