Feeding strategy of mackerel in the Norwegian Sea relative to currents, temperature, and prey

Abstract

High abundance of Northeast Atlantic mackerel (Scomber scombrus L.), combined with limited food resources, may now force mackerel to enter new and productive regions in the northern Norwegian Sea. However, it is not known how mackerel exploit the spatially varying feeding resources, and their vertical distribution and swimming behaviour are also largely unknown. During an ecosystem survey in the Norwegian Sea during the summer feeding season, swimming direction, and speed of mackerel schools were recorded with high-frequency omnidirectional sonar in four different regions relative to currents, ambient temperature, and zooplankton. A total of 251 schools were tracked, and fish and zooplankton were sampled with pelagic trawl and WP-2 plankton net. Except for the southwest region, swimming direction of the tracked schools coincided with the prevailing northerly Atlantic current direction in the Norwegian Sea. Swimming with the current saves energy, and the current also provides a directional cue towards the most productive areas in the northern Norwegian Sea. Average mean swimming speed in all regions combined was 3.8 body lengths s 21 . However, fish did not swim in a straight course, but often changed direction, suggesting active feeding in the near field. Fish were largest and swimming speed lowest in the northwest region which had the highest plankton concentrations and lowest temperature. Mackerel swam close to the surface at a depth of 8-39 m, with all schools staying above the thermocline in waters of at least 68C. In surface waters, mackerel encounter improved foraging rate and swimming performance. Going with the flow until temperature is too low, based on an expectation of increasing foraging rate towards the north while utilizing available prey under way, could be a simple and robust feeding strategy for mackerel in the Norwegian Sea.

Geographical expansion of Northeast Atlantic mackerel (Scomber scombrus) in the Nordic Seas from 2007 to 2016 was primarily driven by stock size and constrained by low temperatures

“…To our knowledge, this was the first time mackerel temperature preference was quantified during their summer feeding migration in the Nordic Seas. Other studies have approximated the temperature occupied by mackerel as ranging from 8°C to 14°C, which was supported by our results (Iversen, 2002;Utne et al, 2012, Nøttestad et al, 2016b. 1832 (Hanna et al, 2006).…”

“…Changes in age-and size-structure of the mackerel stock could contribute to variability in distribution range as larger individuals are known to migrate further westward and northward compared to smaller individuals (Nøttestad et al, , 2010(Nøttestad et al, -2015(Nøttestad et al, , 2016b(Nøttestad et al, , 2016c. Mackerel length-at-age has a declined from mid-2000s to 2013 (Olafsdottir et al, 2013) and no drastic changes in age structure of the stock have occurred (ICES, 2016).…”

“…It is a temperate species, which relies on energy reserves collected during summer feeding as energy source for the over-wintering and spawning season (Lockwood, 1988;Olafsdottir et al, 2016). In the Nordic Seas, mackerel feed in the surface mixed layer, typically located in the upper 30 to 40 m and above the thermocline (Godø et al, 2004;Nøttestad et al, 2016b). It is an opportunistic predator with a wide range of prey species (Langøy et al, 2012;Bachiller et al, 2016).…”

“…Main prey is calanoid copepods, and to a lesser extent euphausiids, amphipods, other planktonic crustaceans, pelagic molluscs and fish (Prokopchuk and Sentyabov, 2006;Langøy et al, 2012;Óskarsson et al, 2016;Bachiller, et al, 2016Bachiller, et al, , 2018. Observations show that mackerel frequently occupy temperatures ranging from 8°C 6 to 14°C during the summer feeding period (Utne et al, 2012;Nøttestad et al, , 2010Nøttestad et al, -2015Nøttestad et al, , 2016bNøttestad et al, , 2016c. Nøttestad et al (2016a) suggested that the mackerel expansion in the Nordic Seas was positively related to stock size from 2007 to 2014, when stock size more than doubled.…”

Bioenergetics modeling of the annual consumption of zooplankton by pelagic fish feeding in the Northeast Atlantic

“…During the feeding season NEA mackerel is concentrated in the upper water layer in loose shoals [ 52 ], making standard acoustic methods unreliable for abundance estimation [ 53 ]. Instead, catch–per–unit–effort (CPUE) in kg m -2 from standardized surface trawling was used as a proxy for the total NEA mackerel abundance at stratum st , and 10 m depth as a fixed depth ( d ) for NEA mackerel vertical distribution.…”