Herring impacts on north Norwegian fjord ecosystems

These dramatic events were seen already during the winter of 2011-2012 in the fjords in Troms. Fishing vessels arrived in large numbers, tracking huge schools of Norwegian spring-spawning herring on their way from the Barents Sea feeding grounds to spawning areas off western Norway.

In fact, there were so many herring that the fjord system was clearly impacted: farmed salmon died from lack of oxygen in the water, and humpbacks and orcas feasted on the huge schools of herring. Locals took to the water in small boats and kayaks, the whale-watching industry picked up, and the fjord suddenly became spectacularly crowded in the middle of the polar night.

For the marine science community in Tromsø, the herring and whale invasions raised several questions:

–        Why are the herring coming into the fjords, and how many are there?

–        What impact do they have on the fjord ecosystem, from the water column to the seafloor?

–        How is society affected, both locally along the fjords, slightly further away in Tromsø, and nationally with respect to the fishing industry?

–        And of course, How long will this last?

Fram Centre projects

To answer some of these questions, a group of Fram Centre scientists have developed three projects within the Fjord and Coast Flagship that aim to provide an integrated understanding of the Kaldfjord ecosystem. One project (WHALE) collects baseline hydrographic and water chemistry data during monthly sampling campaigns. Short-term organic flux data are collected using 24-hour sediment trap deployments. In addition, WHALE implements two hydrographic models; predicted circulation patterns are compared and validated with field data from hydrographic surveys and moored current meters.

This combination of models and field measurements will enable us to look at water exchange between the fjord and the deeper waters off the shelf and estimate how quickly the fjord waters are renewed. Supplemented with samples taken already in autumn 2016, and oxygen profiles taken during the hydrographic surveys, this study will provide clues as to how herring superabundance affects water chemistry.

In context

The EFFECTS project investigates the fate of organic matter that sinks to the seabed, including herring killed or injured by fishing and predation. We place these results in the context of a larger study of geochemical processes at the seafloor to understand how the appearance of large herring schools affects ecosystem processes in north Norwegian fjords.

Scavenging rates on herring carcasses in Kaldfjorden proper were 10-30 times higher than in nearby Vengsøyfjorden. Primary scavengers in Kaldfjorden were large snails and Atlantic cod. The residence time of carcasses on the fjord bottom has a significant impact on sediment processes and the oxygen content of bottom-water.

weShare

The third project, weShare, addresses herring and their predators (including whales and fishing fleets). Frequent hydroacoustic surveys during the herring overwintering season assess their biomass and distribution within the Kaldfjord–Vengsøyfjorden system. We also investigate interactions between herring and their whale predators by using dive recorders and accelerometers attached to the whales. These instruments detect feeding lunges which, when coupled with population sizes, can help us estimate how much herring is consumed by the whales.

In addition, humpback and orca migration routes are mapped using either GPS loggers or satellite telemetry, and these data are coupled with photo-identification of individual whales.

400-600 humpback whales

Preliminary results suggest that 400-600 humpback whales visit these fjords during a season and that some individuals remain in the area for 4-6 weeks. We estimate that humpbacks consume roughly 20 000 to 25 000 tonnes of herring within a season. In comparison, fisheries catch during the 2014/2015 season in the Kaldfjorden–Vengsøyfjorden system was approximately 38 000 tonnes.

Our initial estimates of the total herring biomass within this fjord system during that season indicate a peak biomass of ~1 500 000 tonnes in mid-December, which represents about 20% of the entire spawning stock of Norwegian spring-spawning herring.

A multi-disciplinary perspective

Together, these three projects describe how mass occurrences of herring affect water chemistry, dissolved oxygen levels, and vertical flux of organic matter in Kaldfjorden. We evaluate the various potential fates of herring, from direct predation and fishing loss, to scavenging and decomposition on the seafloor.

Companion projects study the socioeconomic impacts of seasonal herring and whale visits to north Norwegian fjords. Thus, we are developing a multi-disciplinary perspective on the effects of herring on a north Norwegian fjord.

Leaving?

After six straight years of herring superabundance in Kaldfjord, no large schools appeared there in late 2017. While this is disappointing for the completion of these projects – not to mention the local tourist and fishing industries – we have compiled enough data to begin to piece together the multi-level effects of herring on the ecosystem, and on the biology of their whale predators. Recent shifts in winter abundances of herring may, in fact, broaden the implications of this research.

It appears now that many areas along the coast can experience winter herring invasions. The data gathered here can help predict the impact of these events on fjord ecosystem processes across the fjords of north Norway in general. The results are valuable for managing herring as a biological resource, and for addressing potential synergies and conflicts among industries associated with their presence, informing managers of both threats and benefits of the winter wanderings of this important fish population.