Seasonal variability of high-latitude foraging grounds for Atlantic bluefin tuna (Thunnus thynnus)

Abstract

Aim

Predicting spatial and temporal changes in species distributions is difficult for highly mobile species, making effective management challenging. We aim to understand the seasonal variability in habitat suitability for Atlantic bluefin tuna (Thunnus thynnus; ABFT) by using tracking data to create species distribution models.

Location

North-east Atlantic Ocean.

Methods

Forty-nine ABFT were tagged with pop-up archival satellite tags along the west coast of Ireland during successive autumns from 2016 to 2021. Four thousand two hundred and sixteen daily locations were used to build a habitat model to describe ABFT environmental preferences and habitat suitability. For each location, 100 ‘pseudoabsence’ locations were simulated using correlated random walks.

Results

Tags had a mean retention rate of 237 ± 107 days, resulting in 11,602 cumulative days at liberty, and a mean daily distance travelled of 51 ± 143 km. ABFT showed two primary migration routes from Ireland, travelling to the Bay of Biscay or across the Mid-Atlantic ridge in the winter. However, in 2019 six individuals travelled northwards towards Iceland, likely in response to a marine heatwave. Eight ABFT entered the Mediterranean in May/June for putative spawning, with six returning to Irish waters a year later. Five environmental covariates best described ABFT habitat, with habitat suitability highest at sea surface temperatures (SST) of 15–18°C, a mixed layer depth between ~200 and 300 m, high relief bathymetric features (i.e. shelf breaks and seamounts) and frontal regions as revealed by non-linear preferences of eddy kinetic energy and SST standard deviation.

Main Conclusions

In the last decade, ABFT have been re-establishing historic foraging grounds in higher latitudes, for example, Irish and Nordic waters. Our model shows the extent of suitable habitat in these regions and how recent ocean warming may have contributed to northern expansions. Understanding the drivers behind these reappearances is fundamental to successful future management.

Achoimre

Aidhm

Tá sé deacair athraithe i ndáileadh ama agus spásdháileadh speiceas atá an-soghluaiste a thuar, rud a chiallaíonn go bhfuil sé dúshlánach bainistiú éifeachtach a dhéanamh. Tá muid ag iarraidh inathraitheacht shéasúrach in oiriúnacht ghnáthóg an Tuinnín Gorm Atlantach (Thunnus thynnus; TGA) a thuiscint tríd sonraí rianaithe a úsáid chun samhail dháileadh speiceas a chruthú.

Suíomh

Oirthuaisceart an Atlantaigh.

Modh

Rinneadh clibeadh ar 49 TGA le clibeanna cartlainne a thagann go barr uisce ar chósta iarthar na hÉireann le linn ceithre fhómhair i ndiaidh a chéile ó 2016–2021. Úsáideadh 4216 suíomhanna laethúil chun samhail ghnáthóg a chruthú a rinne cur síos ar roghanna timpeallachta agus oiriúnacht ghnáthóg an TGA. I gcomhair gach suíomh, ionsamhlaíodh 100 suíomhanna bréag-neamhláithreacht ag úsáid siúlóidí randamacha comhghaolaithe.

Torthaí

Bhí meán ráta coinneála de 237 ± 107 lá ag na clibeanna, rud a thug 11,602 lá taifeadta i ndiaidh a chéile agus meán fhad laethúil taistealta de 51 ± 143 km dá bharr. Léirigh TGA dhá phríomhbhealaí imirce ó hÉirinn, ag taisteal go Bá na Bioscáine nó thar Droim an Atlantaigh Láir i rith an gheimhridh. Ach i 2019 thaisteal sé iasc ó thuaidh i dtreo an Íoslainn, is dócha mar gheall ar thonn teasa mara. Chuaigh ocht TGA isteach sa Mheánmhuir i rith mí Bealtaine/mí an Mheitheamh chun uibheacha a sceitheadh de réir dealraimh. Tháinig sé dóibh ar ais go huiscí na hÉireann an bhliain dar gcionn. Déanann cúig athróg timpeallachta an cur síos is fearr ar ghnáthóg an TGA, tá oiriúnacht ghnáthóg is airde ag teocht barr uisce (TBA) de 15–18°C, nuair atá doimhneacht an chisil mheasctha idir ~200 agus 300 m, ag gnéithe bataiméadrach ard-rilíf (i.e. scoilteanna scairbhe agus tuláin grinnill), agus ag réigiúin frontacha léirithe ag roghanna neamh-líneacha fuinneamh cinéatach guairneánach agus diall caighdeánach TBA.

Príomhchonclúidí

Le leathscór bliain anuas tá TGA tar éis réigiúin sealgaireachta stairiúil a athbhunú in áiteanna le hard-domhanleithead, ar nós uiscí na hÉireann agus uiscí Nordacha. Taispeánann ár samhail achar an ghnáthóg oiriúnach sna réigiúin seo agus an bhaint féideartha a bhí ag téámh aigéanach le gairid ar leathadh ó thuaidh. Tá sé buntábhachtach na cúiseanna don athfhilleadh seo a thuiscint i gcomhair bainistiú rathúil amach anseo.