bluefin-tuna-feeding-habitat

Bluefin Tuna Feeding Habitat Forecast

logo Issued 17-09-2018. Valid through to 31-08-2019.

Prepared by Mark R. Payne, DTU Aqua, Copenhagen, Denmark.

This is the first forecast for the 2019 feeding season. The next update is planned on or before 15 October 2018.

Forecast

All forecast models suggest the absence of suitable habitat for bluefin tuna in the Denmark strait region in August 2019 (Figure 1): however, the models forecast expanded habitat around the Rekyjanes ridge area and in the central Norwegian sea when compared to the long-term mean. The total area of thermally suitable habitat in this region will be lower than the peaks seen in 2010 and 2012 (Figure 2), although the amount of habitat is expected to remain above that seen prior to the mid-1990s.

**Figure 1. Probability of a given region being thermally suitable feeding habitat  for bluefin tuna in August 2019.** *For each pixel, the probability of the sea surface temperatures being thermally suitable for Atlantic bluefin tuna (i.e. above 11 degree C) is estimated from the North American Multi-Model Ensemble (NMME). The contour line corresponding to a probability of 0.5, which has been used here to define the most likely limit of suitabe habitat, is plotted  as a heavy blue line, while the average habitat boundary (1983-2005) is plotted with a red line. All regions south and east of these lines are classified as suitable habitat.*

Figure 1. Probability of a given region being thermally suitable feeding habitat for bluefin tuna in August 2019. For each pixel, the probability of the sea surface temperatures being thermally suitable for Atlantic bluefin tuna (i.e. above 11 degree C) is estimated from the North American Multi-Model Ensemble (NMME). The contour line corresponding to a probability of 0.5, which has been used here to define the most likely limit of suitabe habitat, is plotted as a heavy blue line, while the average habitat boundary (1983-2005) is plotted with a red line. All regions south and east of these lines are classified as suitable habitat.

**Figure 2. Time series of the area of suitable habitat.** *Interannual variability in the area of water warmer than 11°C during August in the Denmark Strait – Irminger Sea area east of Greenland is plotted from 1960–2020. The area of water >11 °C was estimated within the region 55–70°N and 50 –10 °W (see Figure 3). In addition, the plot also shows the forecast area of habitat for August 2019 based on the forecasts from North America Multi-model Ensemble (NMME)  shown in Figure 1: the width of the area is related to the density of the predictions, while the heavy red dot is the ensemble mean prediction.*

Figure 2. Time series of the area of suitable habitat. Interannual variability in the area of water warmer than 11°C during August in the Denmark Strait – Irminger Sea area east of Greenland is plotted from 1960–2020. The area of water >11 °C was estimated within the region 55–70°N and 50 –10 °W (see Figure 3). In addition, the plot also shows the forecast area of habitat for August 2019 based on the forecasts from North America Multi-model Ensemble (NMME) shown in Figure 1: the width of the area is related to the density of the predictions, while the heavy red dot is the ensemble mean prediction.

Background

An exploratory fishing survey for Atlantic mackerel performed in August 2012 in Denmark strait between Iceland and Greenland captured three individual adult bluefin tuna (Thynnus thunnus) (MacKenzie et al. 2014). It is believed that this was the first such reported occurrence in this region in at least 370 years, and most likely ever. The tuna were captured in a single net-haul in 9-11º C water together with 6 tonnes of mackerel, which is a preferred prey species and itself a new immigrant to the area (Astthorsson et al., 2012). Regional temperatures in August 2012 were historically high and part of a warming trend since 1985, when temperatures began to rise. As a consequence, the area of relatively warm water (i. e., > 11 C) which could serve as potential habitat in this region has expanded by more than 800 000 km2, an area the size of France (Figure 2). The presence of bluefin tuna in this region is likely due to a combination of these warm temperatures that are physiologically more tolerable and immigration of an important prey species into the region.