A feisty shark in Norwegian waters – the tale of the spurdog

Research notes

Marine predators are key species in many ecosystems and can function as indicators of food web health. The spurdog (Squalus acanthias) is a small coastal shark that can be found in temperate and boreal waters around the world. This shark, also known as spiny dogfish, has an interesting tale to tell.

By: Claudia Junge, Ole Thomas Albert and Marlén K Myrlund // Institute of Marine Research, and Maja K Rodriguez Brix // The Norwegian Directorate of Fisheries

Spurdogs form large schools, which means that they can be caught in large quantities, once encountered by fisheries. Males and females often form their own schools, as do large and small fish. Females give birth to a small number of live offspring after two years of pregnancy, one of the longest gestation periods known among vertebrate species. Therefore, capturing large schools of pregnant females has a significant effect on future recruitment levels. For these reasons, spurdog, like many other shark species, is considered particularly vulnerable to overexploitation.

Spurdog fishery – past and present

The northeast Atlantic spurdog fishing stock has undoubtedly been very large and has provided a basis for valuable fishing for over a hundred years.

It has long been sought after for its liver oil and meat. After several decades of overfishing, with annual landings peaking in the 1950s/60s 30 000–60 000 tonnes, the stock size reached a historic low in the early 2000s. At that time, the stock biomass was only about 20% of the previous level. After that, stricter management measures were introduced, and the stock has since been increasing. Major fishing nations were France, the United Kingdom, and Norway.

There is currently no targeted fishery on this stock, and according to the latest assessment by the International Council for the Exploration of the Sea (ICES), the stock has not been overfished since 2005. The population now seems to be on the path to recovery, though according to the current assessment model it will probably be more than 20 years before spurdog numbers reach the desired level. However, data on important life history parameters used in the assessment of this stock, such as growth, fecundity, as well as the population’s sex and age composition, originate mainly from the period before the stock collapsed. The paucity of recent knowledge on those critical life history parameters for the entire population and the complete lack of such data for Norwegian waters was the motivation for our comprehensive study investigating 3 948 individual spurdogs from along the Norwegian coast.

Spurdog is currently landed in Norway mainly at landing sites in the southern half of the west coast. While spurdog is landed in every month of the year, there are two main landing periods: one in spring (April–May) and one in winter (September–January). According to fisher organisations, the prevalence of spurdog seems on the rise, and more catches have been reported from different areas of the country. Since 2011 the annual Norwegian landings have been stable at 216–313 tonnes, which is significantly more than other countries.

Important life history parameters

We analysed whole specimens collected from landing sites along the Norwegian coast between 2014 and 2018; most had been caught by gillnet (87%). We dissected the specimen, collecting a large amount of data and samples. We measured the spurdog’s total length and weight and determined its sex and sexual maturity stage. In addition, we measured sexual characteristics of female and male reproductive organs, and collected samples such as spines, stomachs, vertebrae and tissue. This is a somewhat messy job, but it provides a lot of important data, ensuring optimal use of captured spurdog individuals, to yield the greatest possible knowledge gain.

2.

Spurdogs – a k a spiny dogfish – have spines in front of their two dorsal fins (on their back), which is great, as we can use those to figure out how old an individual shark is. We counted the number of growth bands on the second dorsal spine, similar to the way tree rings are counted, to determine the age of each sampled spurdog. Using spines for aging was shown to be more reliable than using the shark’s vertebrae. The annual deposition of growth bands in the enamel of the spine has been validated before, confirming a correlation between the number of rings and the age of the shark. However, recent research shows that the older a shark is, the less its growth bands might correspond to its age, which means that this type of aging data might tend to underestimate especially the older ages.

In our study, we found males and females ranging in age from 3 years to their mid-30s, but most individuals were less than 15 years of age. Their length varied from 41-95 cm for males and 53-121 cm for females. The youngest and smallest sexually mature females were 7 years old, and the oldest and largest immature ones were 26 years old, giving a mean maturity age of 15.3 years in Norwegian waters. Our study shows that spurdogs use Norwegian coastal waters for their whole life cycle.

Some good news on population recovery

Our research shows that younger age groups are currently dominating the spawning stock, due to an increase in recruitment of “young adults” which are those sharks not fully recruited to the stock until after the ban on the direct fishery. In addition, our analysis indicated a much steeper increase in year-class strength for this series of year classes than estimated in the current ICES assessments, and, therefore, the potential for a much swifter recovery of the spurdog stock.

The importance of fishery for population development is strongly dependent on which parts of the population are fished. Spurdog, like many other shark species, is considered particularly vulnerable to overexploitation and therefore needs to be managed carefully. However, it is also often regarded as a problem species: because of its abundance, its spines, and its sandpaper skin, it can create problems for fishing for other species. With increased knowledge of the catch composition, important life-history parameters and how the stock utilises Norwegian waters, more targeted management measures can be implemented, such as area and seasonal restrictions. Such restrictions affect other fisheries to a lesser extent and at the same time protect the spurdog stock, ensuring its continued recovery.