Population ecology of mesopelagic fishes

This thesis focuses mainly on mesopelagic fishes, and mesopelagic fish species Maurolicus muelleri. Mesopelagic fishes are found in the oceanic twilight zone starting at the end of the euphotic zone down to approximately 1000 meters. The mesopelagic zone is found in all world oceans, and the fishes that inhabit them potentially occupy the highest vertebrate biomass on earth, dwarfing the annual commercial fishery landings. Their low trophic level and high abundance suggests that they can become important as a sustainable source of protein and lipids for an increasing human population. However, they play a part in active carbon sequestering, which means overexploitation can possibly have disastrous consequences. Mesopelagic fishes are also important ecologically as the mid-trophic link between primary production and commercial species.

In this thesis, I have used a lowered hydro acoustic observation platform to identify and separate mesopelagic fishes from other taxa. I have used the a split beam echosounder to provide three dimensional locations of the target to track and quantify behaviour of common mesopelagic fish Maurolicus muelleri (pearlside) and their prey Calanus finmarchicus, and my findings are relevant for mesopelagic fish ecology, abundance estimation and potentially carbon binding.

Mesopelagic fish and physonect siphonophores have similar backscattering properties. By observing the backscatter of both siphonophores and fish at Vøringplatået at the coast of Norway over a from 35 to 160 kHz with some of the targets visually identified by a photo camera , I have shown that siphonophores species Nanomia cara likely can be separated from pearlside. This is because Nanomia cara’s observed backscattering signature obtained with broadband echosounders differed from pearlside. This can provide more precise biomass estimates in the future, and highlights the importance of alternative ground-truthing methods to verify acoustic data. However, there are still large challenges in the ground truthing process that needs further work.
The three-dimensional position data provided by the split beam echosounder can also be used to observe behaviour. By observing both juvenile and adult pearlside at close range in Sørfjorden in Norway and tracking them for more than 60 seconds I found that some of the fish showed specific swimming patterns. The 3D position data I obtained were used in a path geometry model were I could quantify the degree of overlap within the tracks. By fitting the trajectories to the path geometry model, I saw that the juvenile pearlside fish took larger risks near the surface compared to the overwintering adults in the deeper water layer. Some of the mesopelagic fishes moved in a way which enabled efficient foraging within their own visual range, while still staying hidden within the visual range of the predators.

Studying interactions with their preferred prey, I managed to resolve 3mm long copepods Calanus finmarchicus as single target tracks with the echosounder and observe their behaviour in three dimensions. By applying the self-overlap model, previously only applied on copepods in laboratories, and calculating their numerical density, and estimate relative light extinction, I learned that the C.finmarchicus in the surface took higher risks compared to two other depths. Their behaviour were likely a compromise between food search and anti-predator behviour, while behaviour in the intermediate depths were more convoluted. Higher light intensity and higher degrees of copepod movement led the fish to migrate to a lower density of prey to forage.