Avian Predation
Social media has been abuzz lately with discussions surrounding Goosanders and Fish Eating Birds. Chris White, a dedicated trustee of the North Wales Rivers Trust and life long angler, has tirelessly championed this cause for the past two decades. He is now pleased to witness new faces finally entering the conversation. In the following article, Chris White offers his insights on the matter of bird predation.
‘One thing that seems to unite anglers is concern over the impact of increasing numbers of Goosanders and Cormorants on our rivers. Unfortunately, anglers consider that this should be dealt with by Rivers Trusts, this is not the role of Trusts who are conservation bodies. Within Wales anglers should support the Campaign for the Protection of Welsh Fisheries (CPWF www.cpwf.co.uk ) who can, and do, lobby on behalf of angling organisations. The CPWF conservation officer has written to the Minister on more than one occasion about this issue.
The North Wales Rivers Trust is as concerned about avian predation as anglers and as such is preparing to survey spawning streams over the winter period to assess the numbers of FEB’s feeding on spawning streams. We propose to conduct the survey using a drone, overflying spawning streams and then analysing the video to count FEB’s in this way we will gather objective evidence of the potential damaged being caused over the winter period to fry/parr. The general scientific opinion is there is only an issue when smolts are migrating, this ignores the fact that there will be fewer smolts to migrate due to predation on spawning streams over the winter period.
The following graph is taken from a Wetland Bird Survey (WeBS) survey which uses data from the British Trust for Ornithology (BTO) Waterbirds in the UK 2018/19. Note the trend in this graph is upwards but this includes overwintering birds. (this graph uses an index and not numbers of birds)
According to the BTO an adult goosander needs to consume 400gm of fish per day and to raise a single chick from egg to adulthood requires 33KG of fish (goosanders typically have 10 chicks to feed) – these birds only eat fish (and maybe some invertebrates) to live.
BTO Status summary on goosanders:-
Goosanders were first discovered to have colonised the UK in Perthshire in 1871 and spread from Scotland into northern England in the 1940s. Between the first two breeding atlases, the species expanded its range in northern England, and colonised Wales and southwest England. WBS samples became large enough for annual monitoring in 1980, and showed sustained population increase, apart from a slight dip in the late 1990s. The BTO's two national surveys of sawbills demonstrated an average increase in population size of 3% per annum between 1987 and 1997. There has been considerable further range expansion since 1990. Reasons for the colonisation of the UK, and the subsequent range expansion and population increase, are unknown. The species' winter trend in Britain, comprising British breeders and continental visitors, rose steeply from the late 1960s and peaked in the mid 1990s, before falling back, and now stands at early 1990s levels).
To determine the biomass consumed by goosanders and to keep the arithmetic simple consider 100 goosanders feeding over the winter period from November to March approx 126 days. Assuming each goosander consumes on average 400gm of fish per day then over 126 days 100 goosanders could consume approximately 5 Tonne of fish during this period. Admittedly theses birds may not feed every day but they tend to gorge feed so allowing for none feeding periods let us say these birds consume 3 Tonne of fish. On rivers which do not have coarse fish present they will only be eating salmonids (and maybe minnows). A typical smolt ready for migration is approx 100gm but during the winter period most fish eaten will be parr or fry with a likely average weight of 30gm which would indicate that the number of fish eaten is circa 100,000. We will refer to these as smolt equivalents. Over the winter period goosanders can be found feeding on upland spawning streams i.e. feeding on smolt equivalents and once the food source is depleted in one stream they simply fly to the next. No doubt it will be claimed there is no evidence to demonstrate harm from goosanders to ‘fish stocks’. I use the term fish stocks as goosanders will also feed on rivers containing coarse fish (and the sea) and by including these in the biomass consumed it is easy to show there is little damage to overall fish stocks. This is not the case on rivers which do not contain coarse fish.
The focus of NRW and other agencies in the UK has been on avian predation during smolt migration on the basis that a smolt lost cannot be replaced whereas a parr lost can be! This comes from the theory that it is bigger parr that smolt and these take up the best feeding stations, if these larger parr are eaten then their place will be taken by a smaller parr so there is no problem. I am not sure if this is a logical statement, but it seems to be the one that ‘scientific opinion supports’ and they therefore only investigate predation during smolt migration after the recruitment of juveniles have been decimated over the winter months.
The conflict we are grappling with revolves around two protected species: the Goosanders, which, while not endangered and non-native, have successfully naturalized in the UK, and the salmon and sea trout, which are considered 'at risk.' This complex situation necessitates a delicate balance between conservation efforts.
It is evident that without considering the biomass consumed by Fish Eating Birds (FEBs), the prospects for the recovery of salmon and sea trout stocks remain bleak. The North Wales Rivers Trust is committed to addressing this issue by meticulously collecting objective evidence of FEBs' feeding activities on spawning streams throughout the winter season.
Our primary objective is to provide clear and irrefutable data that may shed light on the necessity for more effective management measures. It is essential to clarify that it is not within the purview of the Trust to propose specific controls or interventions. Instead, our role is to gather evidence of potential harm, which will then be conveyed to Natural Resources Wales for their expert assessment and consideration.
By taking this evidence-based approach, we hope to contribute to informed decision-making and collaborative efforts that strike an equitable balance between protecting protected species and preserving the vulnerable salmon and sea trout populations in our rivers.’
Chris White- Vice Chairman of The North Wales Rivers Trust