Spring salmon on the River South Esk, Scotland: Scottish Marine and Freshwater Science Vol 7 No 10

Part 5: Geographic exploitation of the coastal net fishery

Objective: What proportion of the catch from the coastal net fishery adjacent to the River South Esk was destined to spawn in the River South Esk?

5.1 Introduction

Net fisheries operating along coastal migration routes of salmon returning to spawn may exploit fish from a number of different rivers (Gilbey et al., 2012). The management of such mixed stock fisheries [defined as those which exploit significant numbers of fish from several river stocks (Anon., 2015 b)] requires a knowledge of the river of origin and the proportion of each river's stock being exploited (Shearer, 1986). Previous work using genetic techniques to investigate stock composition of the northeast English net fisheries (Gilbey et al., 2012) found that levels of genetic differentiation among stocks were sufficient to enable fish to be assigned to rivers within the northeast of England, and to distinguish fish from the northeast of England from fish of Scottish origin. However, the resolving power to assign fish to rivers within Scotland was insufficient to allow robust assignments (Gilbey et al., 2012). Subsequent genetics work using SNPs could also not be used to determine the representation of different rivers stocks in the nets in the present study because, again, there was not enough differentiation detectable among the large Scottish east coast rivers using the techniques that are currently available.

Numerous tagging studies have shown that salmon caught and tagged at a particular point along the coast may be returning to a variety of rivers (e.g. Shearer, 1986; 1992; Smith & Johnstone, 1996). The radio tracking data used to determine the spawning locations of spring salmon within the River South Esk (see Section 2) were used to estimate the proportion of the catch from the coastal net fishery that was destined to spawn in the River South Esk.

5.2 Methods

This portion of the investigation concerns only those fish caught and radio tagged from the coastal net fishery in 2012 ( n = 153) and 2013 ( n = 38). These fish were processed in the manner described in Section 2.2. Radio tagged salmon sourced from the coastal net fishery in 2012 and 2013 were assigned to the River South Esk stock using a decision tree ( Appendix 5.5).

5.3 Results

Rivers in which tagged fish were detected

In 2012, tagged fish sourced from the coastal net fishery were detected in five of the seven rivers on the east coast of Scotland being monitored in that year (Rivers Don, Dee, North Esk, South Esk and Tay; Figure 5.3.1 a). No tagged fish were detected in either the Bervie Water or the Lunan Water in 2012. A total of 3 fish were recaptured at sea having not been detected in a monitored river, and a further 4 fish were taken by the net and coble fishery in the River North Esk, having first been detected in that river ( Figure 5.3.1 a).

In 2013, tagged fish sourced from the coastal net fishery were detected in four of the eight rivers on the east coast of Scotland being monitored in that year (Rivers Spey, North Esk, South Esk and Tay; Figure 5.3.1 b). No tagged fish were detected in the River Don, the River Dee, the Lunan Water or the River Tweed in 2013. The Bervie Water was not monitored in 2013. Fish were neither recaptured at sea nor taken by the net and coble fishery in the River North Esk in 2013 ( Figure 5.3.1 b).

Figure 5.3.1 a

Figure 5.3.1 b

Estimation of the contribution of the River South Esk stock to the coastal net fishery

2012

In 2012, 12 tagged fish sourced from the coastal net fishery were assigned to the River South Esk stock. 153 fish were tagged in total, of which 3 fish were subsequently caught and killed, leaving 150 fish with the potential to be detected in fresh water. In practice, 48 tagged fish were detected in fresh water after tagging, of which, 12 fish were assigned to the River South Esk stock.

The maximum proportion of fish caught in the coastal net fishery which were South Esk stock assumes that the 48 fish detected in fresh water after tagging were the only ones to survive from tagging to river: 12 / 48 = 25 %.

The minimum proportion of fish caught in the coastal net fishery which were South Esk stock assumes that all 150 fish with the potential to be detected in fresh water survived from tagging to river: 12 / 150 = 8 %.

2013

In 2013, 4 tagged fish sourced from the coastal net fishery were assigned to the River South Esk stock. 38 fish were tagged in total, of which 0 fish were subsequently caught and killed, leaving 38 fish with the potential to be detected in fresh water. In practice, 14 tagged fish were detected in fresh water after tagging, of which, 4 fish were assigned to the River South Esk stock.

The maximum proportion of fish caught in the coastal net fishery which were South Esk stock assumes that the 14 fish detected in fresh water after tagging were the only ones to survive from tagging to river: 4 / 14 = 29 %.

The minimum proportion of fish caught in the coastal net fishery which were South Esk stock assumes that all 38 fish with the potential to be detected in fresh water survived from tagging to river: 4 / 38 = 11 %.

Movements of fish among rivers

For tagged fish sourced from the coastal net fishery, entry into a given monitored river and subsequent return to the sea, occasionally followed by entry into a different monitored river, was a feature in both 2012 and 2013.

Of the 48 tagged fish sourced from the coastal net fishery that were detected in fresh water in 2012, 13 fish (27.1 %) subsequently returned to sea. Of these, 1 fish (7.7 % of those fish that returned to sea having been detected in a monitored river) was subsequently detected in a different monitored river from that in which it was first detected - having initially entered the River North Esk, this fish returned to sea and entered the River South Esk where it remained. This tag was no longer associated with a live fish at spawning time. The fate of the remaining 12 fish that returned to sea having been detected in a monitored river was unknown.

Of the 14 tagged fish sourced from the coastal net fishery that were detected in fresh water in 2013, 5 fish (35.7 %) subsequently returned to sea. Of these, 2 fish (40.0 % of those fish that returned to sea having been detected in a monitored river) were subsequently detected in a different monitored river from that in which they were first detected - 1 fish was initially detected in the River North Esk before returning to sea and subsequently entering the River Tay; and 1 fish was initially detected in the River South Esk before returning to sea and entering the River North Esk. This fish then returned to sea again, entered the River South Esk and finally returned to sea, after which it was not detected again. The fate of the remaining 3 fish that returned to sea having been detected in a monitored river was unknown.

5.4 Discussion

Tagged fish sourced from the coastal net fishery were detected in several major rivers in the northeast of Scotland: the Rivers Spey, Don, Dee, North Esk, South Esk and Tay. In addition, it is possible that fish destined for rivers other than those being monitored could have been intercepted by the coastal net fishery near the mouth of the River South Esk. Therefore, the number of rivers from which fish were intercepted should be viewed as a minimum estimate. The proportion of the catch from the coastal net fishery destined to spawn in the River South Esk was estimated at 8 % to 25 % in 2012 and at 11 % to 29 % in 2013.

Aerial surveys were also carried out on the Lunan Water and the Bervie Water and no tags were detected in these (relatively small) rivers. This is not surprising given the low numbers of salmon reported being caught in these rivers during the months up to and including May in recent years. Using a time series of total reported rod catch of wild salmon and wild grilse (retained and released) for the Lunan Water (1993 to 2012) and the Bervie Water (1960 to 2012) (Scottish Government, 2013) reveals that the months up to and including May account for just 8.9 % of the reported catch in the Lunan Water (4 fish out of a total of 45 reported for the time series as a whole) and just 2.5 % of the reported catch in the Bervie Water (11 fish out of a total of 441 reported for the time series as a whole). Furthermore, no tagged fish sourced from the coastal net fishery in 2013 were detected in the River Tweed.

The tracking data provide direct observations of the subsequent spawning migrations undertaken by spring fish tagged in the coastal net fishery. However, such tracking data has limitations which must be considered when using such data to estimate the composition of the coastal net fishery. Firstly, not all rivers that tagged salmon could possibly enter were monitored for tags. Secondly, there are no estimates of the losses of fish between tagging and subsequent river entry. Thirdly, using radio tracking data to assign fish to a specific river stock relies on the tagged fish being detected again after release, and only about one third of the tagged fish that were sourced from the coastal net fishery were detected again after tagging. Factors such as post-tagging mortality and other sources of post-sampling loss would also influence a mixed stock fishery assessment made from tracking data. Furthermore, the likelihood of tagging a fish from a particular river will be strongly influenced by the river stock size and perhaps differences in coastal migration patterns among river stocks.

When assessing the likelihood of intercepting Atlantic salmon destined for other rivers, it is important to consider their geographic proximity to the point of tagging (e.g. Shearer, 1986) in light of migration routes along the coast (Mills, 1989; Malcolm et al., 2010). For example, the lower proportion of tagged salmon entering the Aberdeenshire Dee in the study by Smith & Johnstone (1996) relative to that reported by Smith et al. (1994) probably reflects the greater distance of the point of tagging from the river mouth (Smith & Johnstone, 1996).

For tagged fish sourced from the coastal net fishery, entry into a given monitored river and subsequent return to sea, sometimes followed by entry into a different monitored river, was a feature in both 2012 and 2013, with 27.1 % and 35.7 % of the fish detected in fresh water in 2012 and 2013 respectively exhibiting such behaviour. One possible explanation for this behaviour is that fish destined for a particular river stray into another river before dropping out of the system and presumably heading for their intended river. Another explanation is that such fish enter a particular river and find conditions within the river to be unsuitable or intolerable to them at the time and therefore drop out of the system. This phenomenon has been observed previously. For example, Stewart et al. (2009) noted that among over-summering Atlantic salmon returning to rivers in the Cromarty Firth in northeast Scotland, 50 % of fish which entered rivers subsequently dropped back to the estuary and ascended adjacent rivers. Stewart et al. (2009) noted that fish residing in non-natal rivers generate mixed stock fisheries such that exploitation in any one river may affect stocks from a range of rivers.

Overall, the radio tracking data show that the coastal net fishery near the mouth of the River South Esk is mixed-stock, catching spring salmon destined for several major rivers in the northeast of Scotland. The proportion of the catch destined to spawn in the River South Esk was estimated at 8 % to 29 %.

5.5 Appendices

Appendix 5.5: Decision tree used to assign radio tagged salmon to the River South Esk stock.