Development of northern pike (Esox lucius) populations in the Baltic Sea, and potential effects of grey seal (Halichoerus grypus) predation
Worldwide, marine mammal populations are increasing after considerable efforts to turn the downward trends caused by hunting, accidental mortality and pollution. The ecosystem effects from the increases of these top predators may be pronounced, but are in most cases poorly known. In the Baltic Sea,...
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| Formato: | H2 |
| Lenguaje: | Inglés sueco |
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SLU/Dept. Of Aquatic Resources
2021
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| Materias: |
| _version_ | 1855572748760252416 |
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| author | Svensson, Rebecka |
| author_browse | Svensson, Rebecka |
| author_facet | Svensson, Rebecka |
| author_sort | Svensson, Rebecka |
| collection | Epsilon Archive for Student Projects |
| description | Worldwide, marine mammal populations are increasing after considerable efforts to turn the downward trends caused by hunting, accidental mortality and pollution. The ecosystem effects from the increases of these top predators may be pronounced, but are in most cases poorly known. In the Baltic Sea, the grey seal (Halichoerus grypus) has been increasing by 5-9% annually since the middle of 1980, with potential effects on fish populations, food webs and fisheries. One possible prey for the grey seal is the northern pike (Esox lucius L.), a large, predatory fish abundant in the Baltic Sea archipelagos. Rcreational fishers are experiencing declines in abundances of pike, and partly blame the decline on seal predation. To assess a potential influence of grey seal on pike population, I have analysed trends in pike abundance and size and correlate it with trends in seal abundance. In addition, I have analysed diet composition of grey seal to study what seals in the archipelago eat. To follow trends in pike abundance I have used data from the Swedish coastal fish monitoring programme and data from fishing competitions. For trends in seal abundance, I used data from the Swedish national grey seal survey. Grey seal diet was estimated from scats and stomach content collected during 2016-2020 from Uppsala County and Stockholm County, in the inner and central parts of the archipelago, i.e. the habitats where pike is most abundant.
Analyses were performed for five counties; Stockholm, Södermanland, Östergötland, Kalmar and Blekinge from 2000 to 2020. The fish monitoring data showed that the abundance of pike smaller than 40 cm has decreased significantly in all counties whereas the abundance of pike larger than 40 cm has decreased significantly in three counties, Stockholm, Östergötland, and Kalmar. The fishing competition data showed that both the abundance and the maximum size of pike have decreased significantly only in Stockholm County. Grey seal abundance has increased in all counties except Kalmar. In the grey seal diet, pike constituted 20% of the diet by weight, after perch (Perca fluviatilis) and herring (Clupea harengus). Overall, there was a negative relationship between the abundance of grey seal and the catch of both smaller and larger pike in the fish monitoring data. There was also a negative relationship between the abundance of grey seal and the maximum size of pike in Stockholm County, while no relationships between the abundance of grey seal and the maximum size of pike were observed in the other counties.
Taken together, the results from this study indicate that the increasing grey seal population is associated with the negatively development of pike population in the archipelagos of the western Baltic Sea. Management of coastal fish has traditionally only taken the effects of fishing into account. The results of this thesis emphasize the importance of a transition towards an ecosystem-based fisheries management, where the effects of increasing populations of top predators are also taken into account in the assessments, to maintain viable populations of fish. |
| format | H2 |
| id | RepoSLU16455 |
| institution | Swedish University of Agricultural Sciences |
| language | Inglés swe |
| publishDate | 2021 |
| publishDateSort | 2021 |
| publisher | SLU/Dept. Of Aquatic Resources |
| publisherStr | SLU/Dept. Of Aquatic Resources |
| record_format | eprints |
| spelling | RepoSLU164552021-02-18T02:01:53Z Development of northern pike (Esox lucius) populations in the Baltic Sea, and potential effects of grey seal (Halichoerus grypus) predation Populationsutveckling hos gädda (Esox lucius) i Östersjön, och potentiella effekter av predation från gråsäl (Halichoerus grypus) Svensson, Rebecka diet analysis, marine mammals predator-prey socio-economic sport fishing recreational fisheries Worldwide, marine mammal populations are increasing after considerable efforts to turn the downward trends caused by hunting, accidental mortality and pollution. The ecosystem effects from the increases of these top predators may be pronounced, but are in most cases poorly known. In the Baltic Sea, the grey seal (Halichoerus grypus) has been increasing by 5-9% annually since the middle of 1980, with potential effects on fish populations, food webs and fisheries. One possible prey for the grey seal is the northern pike (Esox lucius L.), a large, predatory fish abundant in the Baltic Sea archipelagos. Rcreational fishers are experiencing declines in abundances of pike, and partly blame the decline on seal predation. To assess a potential influence of grey seal on pike population, I have analysed trends in pike abundance and size and correlate it with trends in seal abundance. In addition, I have analysed diet composition of grey seal to study what seals in the archipelago eat. To follow trends in pike abundance I have used data from the Swedish coastal fish monitoring programme and data from fishing competitions. For trends in seal abundance, I used data from the Swedish national grey seal survey. Grey seal diet was estimated from scats and stomach content collected during 2016-2020 from Uppsala County and Stockholm County, in the inner and central parts of the archipelago, i.e. the habitats where pike is most abundant. Analyses were performed for five counties; Stockholm, Södermanland, Östergötland, Kalmar and Blekinge from 2000 to 2020. The fish monitoring data showed that the abundance of pike smaller than 40 cm has decreased significantly in all counties whereas the abundance of pike larger than 40 cm has decreased significantly in three counties, Stockholm, Östergötland, and Kalmar. The fishing competition data showed that both the abundance and the maximum size of pike have decreased significantly only in Stockholm County. Grey seal abundance has increased in all counties except Kalmar. In the grey seal diet, pike constituted 20% of the diet by weight, after perch (Perca fluviatilis) and herring (Clupea harengus). Overall, there was a negative relationship between the abundance of grey seal and the catch of both smaller and larger pike in the fish monitoring data. There was also a negative relationship between the abundance of grey seal and the maximum size of pike in Stockholm County, while no relationships between the abundance of grey seal and the maximum size of pike were observed in the other counties. Taken together, the results from this study indicate that the increasing grey seal population is associated with the negatively development of pike population in the archipelagos of the western Baltic Sea. Management of coastal fish has traditionally only taken the effects of fishing into account. The results of this thesis emphasize the importance of a transition towards an ecosystem-based fisheries management, where the effects of increasing populations of top predators are also taken into account in the assessments, to maintain viable populations of fish. Gäddan är en av Sveriges viktigaste fiskart för fritidsfisket. Tack vare den troféstorlek som gäddan kan nå, i kombination med Sveriges orörda natur, vallfärdar många européer årligen för att fånga just sin drömgädda. Men fisket efter gädda i skärgården, som en gång varit känt som världens bästa, har enligt gäddfiskarna förändrats. Fisket efter gädda flyttas mer och mer till sjöarna, och många gäddfiskare förknippar förändringen med predation från det växande gråsälsbeståndet. Syftet med min studie var att undersöka gäddbeståndens utveckling längs svenska kusten, och jämföra dessa med gråsälsbeståndets utveckling. Sälspillning från viloplatser och maginnehåll från skjutna och självdöda sälar analyserades också för att se vad gråsälen i skärgården äter. För att få en bild av gäddbeståndens utveckling längs svenska kusten analyserade jag data från nätprovfiske. Data fanns tillgängligt från Stockholms län till och med Blekinge län. Min analys visar att antalet gäddor har minskat med mer än 90 % i provfisken under 2000-talet, och de troféfiskar som många historier så stolt berättar om finns inte kvar. Samtidigt som bestånden av gädda minskat, breder gråsälen ut sig i skärgården och blir en allt vanligare syn i grunda innervikar, samma vikar som gäddan trivs i. Gråsälen är Östersjöns viktigaste toppkonsument och har just nu det största beståndet på länge med närmare 50000 individer, jämfört med mindre än 4000 individer när antalet var som lägst. När jag analyserar sambandet mellan gråsälens ökande antal och gäddans minskande antal finns ett samband, länsvisa analyser visar på ett samband i alla län förutom Blekinge. Samtidigt bekräftar mina dietanalyser att sett till vikt var gädda gråsälens tredje viktigaste bytesart efter abborre och strömming, i skärgården i Uppsala och Stockholms län. Resultaten från min studie visar att nedgången av gädda har skett i takt med att sälbeståndet har ökat. Traditionellt sätt har fiskförvaltningen längs kusten endast räknat med fiskets uttag. Mina resultat visar på vikten av att ha med effekten av toppredatorer i förvaltningen, och övergå mer till att använda en så kallad ekosystembaserad fiskförvaltning. Kännedom om hur arter påverkar varandra är viktig för att kunna vidta effektiva åtgärder för att stärka bestånden av rovfisk i Östersjön. SLU/Dept. Of Aquatic Resources 2021 H2 eng swe https://stud.epsilon.slu.se/16455/ |
| spellingShingle | diet analysis, marine mammals predator-prey socio-economic sport fishing recreational fisheries Svensson, Rebecka Development of northern pike (Esox lucius) populations in the Baltic Sea, and potential effects of grey seal (Halichoerus grypus) predation |
| title | Development of northern pike (Esox lucius) populations in the Baltic Sea, and potential effects of grey seal (Halichoerus grypus) predation |
| title_full | Development of northern pike (Esox lucius) populations in the Baltic Sea, and potential effects of grey seal (Halichoerus grypus) predation |
| title_fullStr | Development of northern pike (Esox lucius) populations in the Baltic Sea, and potential effects of grey seal (Halichoerus grypus) predation |
| title_full_unstemmed | Development of northern pike (Esox lucius) populations in the Baltic Sea, and potential effects of grey seal (Halichoerus grypus) predation |
| title_short | Development of northern pike (Esox lucius) populations in the Baltic Sea, and potential effects of grey seal (Halichoerus grypus) predation |
| title_sort | development of northern pike (esox lucius) populations in the baltic sea, and potential effects of grey seal (halichoerus grypus) predation |
| topic | diet analysis, marine mammals predator-prey socio-economic sport fishing recreational fisheries |