Evolutionary responses of Atlantic cod (Gadus morhua) to concurrent fisheries and climate stressors

Abstract

Evolutionære ændringer i fisk er efterhånden en anderkendt sandhed, især på grund af forskning i fiskeri-induceret evolution siden 1970'erne. Vi ser især disse ændringer påvirke individuelle livshistorier, resulterende i kortere liv med hurtigere vækst, tidligere modning og mindre asymptotiske størrelser. I tillæg til fiskeri er adaptive ændringer i respons til klimaopvarmning også studeret både teoretisk og empirisk, og responsen hertil afhænger af hvilke temperatur-regimer og populationer der undersøges. Eftersom vi ser en tendens til at arter i varme områder er mindre, og har hurtigere livshistorier, ville vi typisk forvente at øget intensitet af fiskerier samt varmende have ville føre til ensrettede ændringer. Mere nylige studier på den Nordøstartiske Torsk har dog demonstreret at arter som befinder sig i den kolde ende af deres temperatur-niche faktisk bliver større som temperaturen stiger. Hertil kommer at effekten af forskellige typer fiskegrej også kan have vidt forskellige konsekvenser for evolution af livshistorie.

Denne tese udforsker evolution af livshistorier, samt de deraf fremkommende konsekvenser på populationsniveau, ved at bruge Individ-Baseret Modellering af Nordøstarktisk Torsk. Gennem de 3 kapitler heri undersøger jeg mekanistisk de interagerende effekter af varmning samt fiskeri på evolutionær tilpasning.

I Artikel 1 lægger vi en grundlinje for vores nyligt introducerede 'Appetit' parameter, som skal placere denne forskning i kontekst med henhold til evolution af adfærd og personlighed. Vi finder frem til at omdrejningspunktet for evolution af livshistorie i denne model er at fisk søger at nå til store størrelser, modbalanceret af risikoen for at dø inden de gør det. Større fisk er både mere sikre fra predation, og har langt højere relativ fekunditet, og begge disse er eftertragtelige. Som dødelighed stiger, falder sandsynligheden for at vokse sig stor, hvilket gør det mere optimalt at modnes tidligere.

I de efterfølgende kapitler, Artikel 2&3, kommer vi så til sagen, og introducerer varmning og fiskeri som eksterne stressorer. Fiskeri selekterer typisk for hurtigere livshistorier med tidligere modning, men at vælge garnfiskeri (også kendt som gællenet) kan dog selektere for senere modning ved større længde, afhængig af intensitet samt maskestørrelse. Fiskeri resulterer konsistent i en øgning i fodersøgende adfærd og hurtigere vækst. For Nordøstarktisk Torsk øger temperatur også fiskens aerobe kapacitet, hvilket reducerer dødelighed. Dette resulterer i øget vækst, senere modning, større fisk og en generelt større bestand.

Ligesom i Artikel 1 er omdrejningspunktet balancen mellem fordelene ved at være stor, vejet mod chancen for at kunne blive stor. Varmning motiverer fodersøgning, hvilket øger vækst så fiskene kan blive store hurtigere; fiskeri derimod reducerer fordelen ved at være stor, da vi typisk fanger store fisk, og reducerer samtidig også chancen for at fiskene kan nå at blive store. Garnfiskeri lader til at være en mulig undtagelse, da de ikke fanger de helt store fisk, og dermed bibeholder fordelen ved at være stor, såfremt det er muligt at vokse sig op til en sikker størrelse.

Samlet fandt vi at øgende temperaturer har potentiale til at modvirke de evolutionære konsekvenser af fiskeri, men at dette kommer strengt an på hvor intenst der fiskes, hvilket grej der benyttet samt hvor meget varmning der forventes. Evolutionære studier bør betragte begge disse stressorer samlet, da de ellers risikerer at overse interaktioner mellem dem.

Evolutionary changes in fish have by now become a generally accepted truth, in large part due to research into fisheries-induced evolution since the 1970's. We especially see these changes as adaptive alterations to individual life-histories, such as shorter lives with higher growth rates, faster maturation and smaller asymptotic body sizes. In addition to fisheries, adaptive responses to climate warming has also been studied both theoretically and empirically, with results depending on current temperature regime of the population in question. Given the prevalence of smaller individuals with faster paces of life at warmer regions, the Temperature Size Rule would predict uni-directional changes to both increased fishing pressure and warming oceans. However, recent studies on Northeast Arctic Cod have demonstrated that individuals inhabiting the colder ranges of their thermal tolerance may actually see increased sizes in a warming climate. Additionally, research into fisheries selectivity seem to indicate that different gear types may also have different consequences for life-history evolution.

This thesis explores life-history evolution, as well as emergent population level responses, using Individual-Based Modelling of the Northeast Arctic Cod stock. Throughout the three chapters contained within, I aim to mechanistically examine the interacting effects of warming and fishing on evolutionary adaptation.

In Paper 1 the groundwork is laid for the newly introduced 'Appetite' parameter, which places our research into the context of behavioural evolution and personality. We find that a key point of optimisation for life-history evolution, is the desire to reach larger sizes, counterbalanced by the risk of dying before reproducing. Larger fish are safer from predation, and have significantly increased relative fecundity, both of which are desirable. As mortality increases, the probability of reaching these sizes decrease, making earlier maturation at smaller sizes more optimal.

The following chapters, Papers 2&3, then gets to the matter at hand, and introduces warming and temperature as external stressors on the population. Fisheries typically select for faster life-histories with earlier maturation, but opting for gillnets rather than trawling nets is shown to potentially select for later maturation at larger sizes, depending on intensity and targeted sizes. Notably, fisheries consistently result in increased foraging effort and faster growth. For the Northeast Arctic Cod, temperature is found to increase aerobic scope, reducing mortality. This resulted in increased growth, later maturation at larger sizes as well as larger populations overall.

Similarly to Paper 1, the balancing of expected benefits at larger sizes measured against probability of reaching the sizes appear to be the central tradeoff. Warming motivates foraging, which increases growth in order to reach these sizes faster; fisheries can lessen the benefit of reaching larger sizes by selectively targeting large fish, while simultaneously reducing probability of surviving to reach these sizes as well. Gillnets appear to be an exception, by providing a size-refugium, provided it is feasible to grow through the targeted size range.

In conclusion, it was found that increasing temperatures had the potential to (at least partially) compensate for fisheries-induced evolution, though this greatly depend on intensity and method of fishing as well as the level of warming expected. Evolutionary studies would benefit from considering both stressors in unison, as to not overlook potential interactions.