120 National Marine Fisheries Service Fishery Bulletin First U.S. Commissioner established in 1881 of Fisheries and founder NOAA of Fishery Bulletin Abstract—The Atlantic mackerel Changes in the spawning distribution and biomass (Scomber scombrus) has a wide distri- bution off North America that changes of Atlantic mackerel (Scomber scombrus) in the substantially seasonally and interan- nually, challenging attempts to track western Atlantic Ocean over 4 decades changes in stock size. We used egg col- lections to develop an index of spawning David E. Richardson (contact author)1 stock biomass for the southern contin- Lauren Carter2 gent of Atlantic mackerel that spawns 3 in U.S. waters. These results were com- Kiersten L. Curti 2 bined with those from an egg survey of Katrin E. Marancik the northern contingent that spawns Martin Castonguay4 in the Gulf of St. Lawrence, Canada, to develop the only available stock- Email address for contact author: [email protected] wide index. Egg production of Atlantic mackerel in the western North Atlan- 1 3 tic Ocean has dropped >90% from the Northeast Fisheries Science Center Northeast Fisheries Science Center 1980s to the 2010s. The contribution of National Marine Fisheries Service, NOAA National Marine Fisheries Service, NOAA the southern contingent has generally 28 Tarzwell Drive 166 Water Street been around 5–10%, except for during Narragansett, Rhode Island 02882 Woods Hole, Massachusetts 02543 the late 1970s and early 1980s when it 2 Integrated Statistics 4 Maurice Lamontagne Institute was 30–40%. On the continental shelf of 28 Tarzwell Drive Fisheries and Oceans Canada the northeastern United States, there Narragansett, Rhode Island 02882 850 de la Mer Road has been a northeastward shift in the Mont-Joli, Quebec G5H 3Z4, Canada distribution of eggs of Atlantic mack- erel, with low levels of egg production recently at the historically important spawning location off southern New England. Overall, the index of spawn- ing stock biomass generated from sur- veys in the waters of the United States The Atlantic mackerel (Scomber scom- Atlantic Ocean is the cause of these and Canada provides a much needed brus) is a migratory, small pelagic changes (Hughes et al., 2014; Berge means of tracking fluctuations in the species that supports important fish- et al., 2015; Jansen et al., 2016). More biomass of this commercially valuable eries throughout the North Atlantic recently, the assessment has indicated migratory species. Ocean. In recent decades, the major- a decline in biomass of Atlantic mack- ity of worldwide landings of Atlantic erel in the eastern Atlantic Ocean and mackerel has occurred in the eastern the need for quota reductions, present- Atlantic Ocean, where the challenges ing new assessment and management of assessing and managing this eco- challenges. logically important species are well Atlantic mackerel in the west- documented. Specifically, for Atlantic ern North Atlantic Ocean have also mackerel in the eastern Atlantic Ocean, shifted distribution, although the a substantial increase in abundance extent and causes of this shift are less and a shift and expansion in distribu- certain than for Atlantic mackerel in tion started around 2005. This distribu- the eastern Atlantic Ocean, as are tion change led to increased difficulty the effects on resource surveys, stock in surveying the population (Nøttestad assessments, and the availability of et al., 2016), the development of new Atlantic mackerel to regional fisher- Manuscript submitted 12 September 2019. regional fisheries (Astthorsson et al., ies. Atlantic mackerel in the western Manuscript accepted 23 April 2020. 2012), conflicts among nations in quota Atlantic Ocean are considered a sin- Fish. Bull. 118:120–134 (2020). Online publication date: 4 May 2020. allocations (Spijkers and Boonstra, gle population (Sette, 1950), with a doi: 10.7755/FB.118.2.2 2017), and potentially regional changes southern contingent that spawns in in the abundance of mackerel preda- U.S. waters offshore of southern New The views and opinions expressed or tors and prey (MacKenzie et al., 2014; England and in the western Gulf of implied in this article are those of the Óskarsson et al., 2016). The results Maine (Berrien, 1978) and a north- author (or authors) and do not necessarily reflect the position of the National of a number of analyses indicate that ern contingent that spawns in waters Marine Fisheries Service, NOAA. the warming of waters in the North of Canada, primarily in the southern Richardson et al.: Changes in the spawning distribution and biomass of Scomber scombrus in the western Atlantic Ocean 121 National Marine Gulf of St. Lawrence (Sette, 1943; Grégoire1). Results catches despite low landings in both the United States Fisheries Service Fishery Bulletin First U.S. Commissioner from tagging and cohort tracking work, primarily done and Canada, truncated age- structure in the U.S. bottom- established in 1881 of Fisheries and founder NOAA of Fishery Bulletin in the 1930s (Sette, 1950), indicate that these 2 contin- trawl survey, and results of a stock assessment in 2016 in gents mix on the continental shelf of the northeastern Canada indicating that the biomass of the northern con- United States from the late fall through the spring but tingent had reached a historic minimum in 2012 (DFO, then separate during the late- spring spawning season 2017). A joint U.S.-Canada stock assessment conducted in and through much of the summer. 2010 was consistent with this view of the stock (Deroba As with Atlantic mackerel in the eastern Atlantic et al., 2010), but that assessment was not incorporated Ocean, relative catches in different regional fisheries in into management decisions by either nation because of the western Atlantic Ocean have changed over time, in severe retrospective patterns in the assessment model part because of distribution shifts within the population. and unresolved conflicts in the data, in particular between Additionally, across the entire western North Atlantic fishery catches and the index of abundance from the U.S. Ocean, there have been substantial multi- decadal cycles bottom- trawl survey. in landings, with a notable recent 88% decline between One of the long- running challenges in assessing the 2006 and 2015 to the lowest levels since the early 1800s stock of Atlantic mackerel in the western Atlantic Ocean (Hoy and Clark2; NEFSC, 2018). During this decline, only is that the fishery- independent indices that are gener- small portions of the quotas of both Canada and the United ally used for this stock, an egg survey in Canada (1982– States were usually harvested despite industry capacity present) and a bottom- trawl survey in the United States and market demand, leading to questions about why the (1968–present), do not cover the entire range of this stock. industry was underperforming relative to expectations. Therefore, the trends in each index may be affected by Two alternative hypotheses have been proposed to changes in both abundance and availability associated explain the decline in catch of Atlantic mackerel in the with distribution shifts. western Atlantic Ocean over the past 10–15 years. The The egg survey in Canada uses the daily egg produc- first hypothesis is that warming waters in the western tion method (Grégoire et al.4), a well- established approach Atlantic Ocean (Shearman and Lentz, 2010) resulted in that is used throughout the world (Bernal et al., 2012), a shift in mackerel distribution that reduced availabil- including in the multi- nation triennial survey of Atlantic ity to the fishery (Overholtz et al., 2011). This hypothesis mackerel in the eastern Atlantic Ocean (Dransfeld et al., of a change in distribution was supported by the results 2005). For the index of spawning biomass in Canada, a from a bottom- trawl survey conducted off the northeast- systematic plankton survey is used to develop daily egg ern United States, results that included a northeastward production estimates, and gonadosomadic index measure- shift in survey catch and mean abundance per tow near ments from landings are used to evaluate seasonality of the highs of the time series (Overholtz et al., 2011). Simi- spawning to scale daily egg production to annual egg pro- larly, a northeastward shift in landings of Atlantic mack- duction. Fecundity estimates are then used to calculate erel in waters of Canada from Nova Scotia and the Gulf of spawning stock biomass. St. Lawrence to Newfoundland (Grégoire et al.3) indicated Within U.S. waters, there have been sporadic efforts, that the distribution of Atlantic mackerel was changing. each with a slightly different analytical approach, to A stock assessment that estimated high biomass and develop estimates of annual egg production or spawning very low exploitation rates of Atlantic mackerel in 2006 stock size of Atlantic mackerel. These estimates include was consistent with the hypothesis that the population of 64 trillion eggs and 320 million individuals in 1932 Atlantic mackerel was robust but availability to the fish- (Sette, 1943), 18 million individuals in 1966 (Berrien ing industry had declined (NEFSC, 2006). and Anderson5), 303 trillion eggs and 1220 million indi- The alternate hypothesis is that a population decline, viduals in 1977 (Berrien et al., 1984), and 56 trillion rather than a distribution shift, was responsible for the eggs and 240 million individuals in 1987 (Berrien6). The landings decline. This alternate hypothesis was supported 1987 survey was the only analysis to compare egg pro- by the decline in the index of egg abundance for Atlantic duction in the United States and Canada in the same mackerel in the Gulf of St. Lawrence (northern contin- year, with only 10% of stock- wide egg production being gent) (Grégoire et al.4), truncated age-structure in fishery egg surveys conducted in the southern Gulf of St. Lawrence 1 Grégoire, F. (ed.). 2000. The Atlantic mackerel (Scomber scom- from 2008 to 2011. Can.