Lumpfish (Cyclopterus lumpus) spawn in coastal waters in the North Atlantic Ocean, yet little is known about the distribution outside of the spawning

Temperature Proves to Be the Main Driving Factor in Lumpfish Distribution in the North Atlantic

ICELAND
Monday, June 23, 2025, 05:00 (GMT + 9)

New International Study Reveals Crucial Impact of Water Temperature on Lumpfish Location, Especially Outside Spawning Season

REYKJAVIK – Recent research, a collaborative effort between scientists from Iceland, Greenland, the Faroe Islands, Denmark, and Norway, has highlighted that water temperature is the predominant environmental factor influencing the distribution and abundance of lumpfish (Cyclopterus lumpus) in the North Atlantic, particularly during summer. While it was known that this species spawns in coastal waters, its distribution outside the breeding season was poorly understood.

The study, recently published, utilized data from an extensive annual pelagic trawl survey conducted in the Nordic Seas, known as the International Ecosystem Summer Survey of the Nordic Seas (also called the mackerel survey). The results reveal that lumpfish have an extensive distribution, covering the Irminger Sea, the Denmark Strait, and the Norwegian Sea, albeit with a low population density. However, their capture frequency is notably lower south of Iceland and in the North Sea compared to other coastal and oceanic areas.

Figure 1. Map of the Nordic Seas with the main warm (red) and cold (green) surface currents in the Northeast Atlantic (a) and location and density of lumpfish (Cyclopterus lumpus) caught during the International Ecosystem Survey of the Nordic Seas during 2020 (year of greatest coverage) (b). Temperature at 20 m in 2020 is shown. The survey caught two size groups of lumpfish, those smaller than 18 cm and lumpfish which were larger than 18 cm (Figure 2). These two groups were termed juveniles and adults respectively, and their distribution was examined separately. Click on the image to enlarge it

Temperature as a Primary Constraint

The research employed generalized additive models, random forest, and support vector machine to analyze environmental variables. Temperature emerged as a primary constraint on lumpfish distribution. Lumpfish were caught at over 90% of stations where the temperature at 20-m depth ranged between 4°C and 9°C. Capture frequency sharply decreased when the temperature was outside this range. Other environmental variables, such as salinity, mixed layer depth, and measures of primary and secondary production, showed low influence on their distribution.

Figure 2. Length distribution of lumpfish caught during the International Ecosystem Summer Survey of the Nordic Seas between 2010 and 2023.

Differences Between Juveniles and Adults

The distribution of juvenile and adult lumpfish differs significantly. Adults are spread over a larger area than juveniles. The highest concentrations of juveniles were observed in the central Norwegian Sea, but they were also found throughout other areas of the Norwegian Sea, as well as north and west of Iceland. Adults, for their part, were found almost throughout the entire survey area, with the highest density in the northeast and northwest of the Norwegian Sea and around the north and east of Iceland.

Figure 3. The average density of juvenile (< 18 cm) (a) and adult (≥ 18 cm) (b) lumpfish (Cyclopterus lumpus) by 1 × 1° rectangle during the International Ecosystem Summer Survey of the Nordic Seas from 2010–2023 for all years combined. Red indicates the area was surveyed, but no adult or juvenile lumpfish were caught. Click on the image to enlarge it.

The researchers investigated several environmental factors to determine their influence on the distribution of both life stages. Although juveniles were most commonly found at temperatures of 6-10°C, their distribution is most likely determined by a combination of hatching location and ocean currents, showing no preference for waters with higher levels of phyto- or zooplankton. In contrast, temperature was the most important factor for adults, who were most likely to be found at temperatures of 4-8°C. They also showed no preference for areas with high levels of phyto- or zooplankton. The occurrence of both juveniles and adults dropped sharply at temperatures >12°C, which explains their absence from areas south of Iceland and much of the North Sea.

This study underscores the critical importance of temperature in lumpfish ecology, providing valuable insights for the management and conservation of this species in a changing climate.

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