General information
Dipturus batis is the second most abundant skate species in Icelandic waters after starry ray. It is distributed mainly within 200 m depth range and the distribution is mainly confined with the warmer waters off South and West Iceland. Blue skate is rarely caught larger than 100-150 cm. Like other rays in the region Dipturus batis is oviparous i.e. it hatches ferilized eggs embedded in capsules (“mermaid’s purse”). Annual fecundity is believed to be about forty eggs with an extended embryonic development of 9-10 months. Mating is believed to occur in spring and egg capsules are laid in summer. In fall migration between shallow waters (up to 10 m depth) to deeper waters (>100m) is believed to occur. A longstanding taxonomic confusion has existed between the flapper skate (D. intermedius) and blue skate (D. batis) resulting in misidentification and misreporting. Investigation of the common skate complex in Icelandic waters indicated that the dominant species currently found in Icelandic waters is the smaller D. batis now currently referred to as the common blue skate or blue skate (Bache-Jeffreys,2021) and Pálsson & Jakobsdóttir (2018, see below)
The fishery and landings
Like other sharks and skates in this region, blue skate is mainly a bycatch in the Icelandic fisheries. Common skate is fished with a variety of fishing gears with annual landings not exceeding 200 tonnes (Figure 1). They used to be regarded as fairly common in Icelandic waters, but landings may now only be about 20 % of what was landed 50 years ago (Figure 2). A large part of the landed catch is for local consumption, as the species within the common skate complex are traditional food in Iceland, particularly at Christmas time. The remaining catch is processed and mainly exported. The main landings are taken off South-Iceland (Figure 3). Catches off N and NW could be misidentifications and needs more investigation.
Survey data
Distribution and biomass indices
In the surveys IS-SMB and IS-SMH the distribution of blue skate is mainly within the warmer waters off South Iceland (Figure 4). In the gillnet survey blue skate is mainly caught in the coastal area (Figure 5). It is not a frequent catch in any of the MFRI surveys and it is less abundant on the shelf in autumn than in spring. However, occurrence has been increasing in spring survey (Figure 6). The increase is also reflected in increasing trend in the biomass index since 2010 (Figure 7).The mean biomass in annual spring survey is estimated around 600 tonnes.
Life history information
The length range for blue skate in Icelandic surveys ranges from 15- 186 cm. Mean size varies from 44 cm to 78 cm depending on surveys (Table 1). Length distributons are shown for the spring survey (IS-SMB) and the nephrops survey (Figure 8).
Males and females are of similar size and length at 50-%-maturity (L50) is 113.8 and 112.7 for females and males respectively (Figure 9)
Table 1. Blue skate. Length distribution in four MFRI surveys. All data.
| Survey | N | ML (cm) | min (cm) | max (cm) |
|---|---|---|---|---|
| IS-SMB | 1061 | 75.8 | 15 | 186 |
| IS-SMH | 132 | 78.2 | 10 | 160 |
| Nephrops | 869 | 70.0 | 14 | 152 |
| Coastal survey | 80 | 44.0 | 20 | 119 |
References
Bache-Jeffreys, M., de Moraes, B. L. C., Ball, R. E., Menezes, G., Pálsson, J., Pampoulie, C., Stevens, J. R., & Griffiths, A. M. (2021). Resolving the spatial distributions of Dipturus intermedius and Dipturus batis—the two taxa formerly known as the ‘common skate.’ Environmental Biology of Fishes. https://doi.org/10.1007/S10641-021-01122-7
Pálsson, J. and Jakobsdóttir, K..2018. (https://www.researchgate.net/publication/346952621_The_Flapper_or_The_Blue_Dbatis_complex_in_Icelandic_waters)
