Work

Red-Throated Diver Energetics Project - 2019 Field Season Report

Year: 2018

Type: article

Abstract: Offshore wind deployment around the UK is due to increase from the current 8GW to 30GW in the next ten years along with ambitious plans for offshore wind development elsewhere in Europe.Development in the marine environment may affect marine bird populations.Redthroated divers are known to be displaced, causing effective habitat loss, by construction and operation of offshore wind farms, with this effect primarily seen during the non-breeding season when divers use areas of offshore wind development.However, the energetic, physiological and demographic consequences of displacement are currently unknown.If divers are already energetically constrained in the non-breeding season, they may struggle to meet the additional energetic demands following displacement.The aim of the Redthroated Diver Energetics Project (https://jncc.gov.uk/our-work/rtde-project/) is to collect and compare novel data on foraging and movement behaviour of red-throated divers in the breeding and non-breeding season.In 2018, the Red-throated Diver Energetics Project tagged 74 divers breeding in Iceland, Orkney, Shetland and Finland with archival time depth recorder (TDR) and geolocator (GLS) tags.In 2019, tagged divers were found, caught and tags removed to obtain data on their approximate location and their dive behaviour in the 2018/19 non-breeding season.Analyses of these data are presented in a separate report (JNCC Report No. 638).The present report describes the 2019 breeding season, tag retrieval/redeployment and tag/trapping effects on red-throated divers.Recapturing tagged divers was challenging.Locating tagged birds was difficult and catching them took much longer than during tag deployment in 2018.A total of 27 tagged divers were caught and tags removed.Of these, 18 were fitted with new TDR and GLS tags to obtain information on inter-annual variation in foraging behaviour for the same individuals.A further 15 sets of tags were deployed on divers that had not been tagged previously.In 2020, attempts will be made to recapture divers carrying these newly deployed tags plus divers carrying the 2018 deployment that were missed in 2019.Three methods were used to look for any tag and trapping effects: resighting rates of tagged divers, changes in body mass of tagged divers between 2018 and 2019 and breeding success at nest sites where trapping was attempted compared with control sites.Additionally, mortality of three tagged divers was considered in the context of bycatch rates of ringed divers.Across all sites (countries) combined, the resighting rate of tagged divers was reported to be 69% (n=50 tagged divers seen) although this varied from 53% to 92% depending on site.For a long-lived species, a resighting rate of 80-90% would be expected.However, finding tagged divers was challenging and it is possible that tagged birds did return but were not seen.Alternatively, divers may not breed every year, irrespective of whether they are tagged.Without a marked control group of untagged divers, e.g. by using colour rings on untagged divers, it is not possible to make a quantitative evaluation of this resighting rate.Body mass of tagged divers retrapped in 2019 was not significantly different to their body mass in 2018.This suggests that carrying tags does not reduce body condition in individuals that returned to breed.However, it was not possible to look at body mass of tagged birds that were not recaptured so our sample could have been biased.Breeding success was found to be significantly lower at nest sites where trapping was attempted, compared with control nest sites (breeding success = 38% at trapped nests, 58% at control nests for Scotland and Finland, combined), although evidence for this effect in Finland was weak.This suggests that disturbance from attempting to catch divers to deploy or retrieve tags can increase the likelihood of a breeding attempt failing, but that this effect was seen more strongly in some trapping locations than others.Mortality of three Finnish tagged divers was reported during August 2018-June 2019, with two due to drowning in freshwater nets and a third of unknown causes.Bycatch is known to be a problem for adult red-throated divers, e.g.75% and 60% of ring recoveries of Finnishand British-ringed divers, respectively, were due to being caught in a net, where cause of death was known.However, drowning in a net set in freshwater is much more common in Finland compared with Britain (60% and 8%, respectively, of mortalities of known cause for ringed divers).Therefore, these mortalities of tagged divers could have happened irrespective of them carrying tags.Despite breeding success potentially being decreased by attempts to trap divers, and no discernible tag effects (i.e.no decrease in body mass of tagged divers detected) we still recommend trying to recapture and remove as many tags as possible from red-throated divers.It is still possible that ongoing undetected tag effects exist and, given the red-throated diver life history strategy of high adult survival and low productivity, the population will better withstand short-term reductions in productivity from trapping over any small decrease in adult survival due to carrying tags for many years.

Source: Työväentutkimus Vuosikirja