Changes in weight-at-length and size-at-age of mature Northeast Atlantic mackerel (Scomber scombrus) from 1984 to 2013: effects of mackerel stock size and herring (Clupea harengus) stock size

Abstract

Weight-at-length and length-/weight-at-age were analysed for mature 3- to 8-year-old Northeast Atlantic mackerel (Scomber scombrus; n= 26 084) collected annually in autumn (September and October) at the end of the annual feeding season during 1984–2013 in the northern North Sea. The age range represented 92% of the mackerel stock size (age 3+). During the most recent decade, mackerel length- and weight-at-age continually declined. In 2013, the average mackerel was 3.7 cm shorter and weighed 175 g less than the average individual in 2002. Individual weight-at-length, demonstrating annual summer feeding success, continually declined during the most recent 5 years, whereas somatic growth of cohorts aged 3–8 continually declined for the last 11 of 25 cohorts investigated. Growth of the latest cohort was 34% of the maximum cohort growth recorded. Both weight-at-length and cohort growth were negatively affected by mackerel stock size and Norwegian spring-spawning herring (Clupea harengus) stock size (weight-at-length: r2 = 0.89; growth (length): r2 = 0.68; growth (weight): r2 = 0.78), while temperature was not significant. Conspecific density-dependence was most likely mediated via intensified competition associated with greater mackerel density. Negative effects of herring were likely mediated by exploitative competition for shared food resources rather than direct competition due to limited spatio-temporal overlap between mackerel and herring during the feeding season. Herring begin their seasonal feeding migration at least a month before mackerel; therefore, herring consumption influences prey availability for the later-arriving mackerel. Record low mackerel growth and negative effects of mackerel and herring stock size suggest that the carrying capacity of the Norwegian Sea and adjacent areas for plankton-feeding fish stocks have been reached. However, compounding effects of a less productive Norwegian Sea during the 30-year period cannot be excluded.

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

“…At the time of our study, mackerel were distributed from the southern edge of the Norwegian Sea to as far north as 758N (ICES, 2010;Nøttestad et al, 2016). The availability of food should also be expected to have a strong influence on distribution during the summer feeding period (Olafsdottir et al, 2016). The number of samples was limited, but the plankton biomass was nearly twofold higher in the NW than in the SW region.…”

“…Mackerel has recently been recorded as far north as Svalbard (Berge et al, 2015). The recent rapid increase in stock size of mackerel has probably increased the intra-and interspecific competition for limited food resources (Olafsdottir et al, 2016). Increasing temperature (Skagseth and Mork, 2012) and changing prey conditions and concentrations during the last 10 -15 years in the Norwegian Sea Prokopchuk and Sentyabov, 2006;Huse et al, 2012) may have forced mackerel to expand their feeding migration and distribution.…”

“…Increasing temperature (Skagseth and Mork, 2012) and changing prey conditions and concentrations during the last 10 -15 years in the Norwegian Sea Prokopchuk and Sentyabov, 2006;Huse et al, 2012) may have forced mackerel to expand their feeding migration and distribution. Thus, a major driving force behind the great expansion of mackerel in the Norwegian Sea and surrounding waters could be limited available food resources, leading to pronounced density-dependent growth in the mackerel stock (Olafsdottir et al, 2016). A temperature 88C has earlier been suggested to be the lower boundary of the preferred temperature range for NEA mackerel (Iversen, 2002(Iversen, , 2004, which may set the outer borders of maximum distribution of mackerel.…”

Oogenesis and reproductive investment of Atlantic herring are functions of not only present but long-ago environmental influences as well

“…S1 and S2). Although trends in body condition are valuable indicators of fish health and reproductive status under such situations, as for mackerel (16), corresponding RI responses remain unaddressed. In contrast to Atlantic mackerel or blue whiting (Micromesistius poutassou), the third main planktivorous fish in question (SI Appendix, Fig.…”

Opportunistically recorded acoustic data support Northeast Atlantic mackerel expansion theory

“…The SSB of NEA mackerel stock in the last 6 years has been at its highest for the last three decades. This has inevitably led to an increasing pressure on local food resources and, possibly compounded by small numbers of zooplankton prey in some parts of its traditional habitat (Norwegian Sea, ICES 2013d), is likely to have driven this expansion (Astthorsson et al, 2012;ICES, 2013a, Olafsdottir et al, 2016. As temperatures have increased in the north and northwestern extremities of the species' distribution, where colder temperatures were previously a limiting factor (ICES, 2013a), mackerel have been able to expand their range.…”