Dahlke, FT, SN Politis, IAE Butts, EA Trippel, MA Peck. 2016. Fathers modify thermal reaction norms for hatching success in Atlantic cod, Gadus morhua. Journal of Experimental Marine Biology and Ecology, 474(2016): 148-155. doi:10.1016/j.jembe.2015.10.008
There once was a time when people believed in such a large ocean, with so many fish, that it would be impossible to overfish to the extent of endangering fish populations. In their defense, at that time, they were right. However, times have changed. With bigger ships that have stronger engines that can travel farther, faster using modern trawling rigs, we are able to catch A LOT more cod than we used to. GPS, SONAR and RADAR help us locate quality fishing grounds faster and the advent of refrigeration keeps the large catches fresh until fishermen make it back to market.
Our fishing practices tested the notion that humans can’t possibly catch all of the cod in the sea, and we won – the cod lost. Since then, Atlantic cod have been the poster child for overfishing leading to decimated populations. Not all is grim, though. Refreshing news that cod stocks in Newfoundland and Labrador have shown signs of a considerable recovery after decades of decline was released just this past October. This is just one region where cod are found (Figure 1) and historically have been heavily fished, but it is good news none the less.
In addition to overfishing pressures, many marine organisms are threatened by global climate change. Gradually warming oceans present thermal tolerance challenges to fishes adapted for cooler waters. A new paper published in the Journal of Experimental Marine Biology and Ecology has increased our understanding of how Atlantic cod may respond to recovery efforts in warming oceans. The paper addressed paternal effects on an offspring’s ability to tolerate warmer waters during the earliest stages of their development. Atlantic cod on both sides of the Atlantic Ocean prefer to spawn in an optimum temperature around 6 oC. The IPCC expects these ocean regions to warm by 2-4 oC by the end of this century. This study examined how offspring from different fathers may tolerate these temperature increases.
The experimenters ran two parallel sets of experiments: one with cod from the American side of the Atlantic, and one with cod from the European side of the Atlantic. Eggs from one mother were fertilized by 4 different males from the western Atlantic and 5 males from the Baltic region. An equal number of fertilized eggs from each father were incubated at various constant temperatures (Figure 2). Western Atlantic cod were tested at 2.0, 4.0, 6.0, 8.0 and 10.0 oC (again, the optimum range for these fish is ~6.0 oC. This region is expected to reach 8.0-10.0 oC by the end of this century). Due to logistical constraints, the researchers were forced to test the Baltic region offspring at 6.0, 8.0, 10.0 and 12.0 oC.
The two main factors that the researchers were examining in this study were: 1. If paternity affects offspring survival, and 2. If survival rates change unequally to changes in temperature for offspring from different fathers. The latter represents the interaction between paternity and temperature changes. More on that shortly.
The results from cod on the western Atlantic side and the Baltic side were very similar. Paternity effects explained 45-55% of the variation in survival rates. Figure 3 shows an example of what that looks like. Essentially, some fathers produce higher quality offspring that survive better, regardless of the temperature. Frankly, this isn’t horribly interesting. For the most part, we expect there to be a difference in survival depending on which father produced the offspring due to differences in father quality. However, it is important to recognize the idea that survival for all offspring changes in the same way as the temperature changes. Survival is highest at the optimum 6 oC and decreases as the temperature moves away from that optimum. This scenario is what explains about 50% of the differences in survival in this experiment.
More interestingly, the researchers also found a significant interaction between father and temperature. Unlike the previous example in Figure 3, an interaction means offspring from one father may be very negatively impacted by warmer temperatures, while offspring from a different father are not as strongly affected. This is evident by the criss-crossing lines in Figure 4. For example, the offspring of western Atlantic cod father #1 (white circles in Figure 3) had lower survival rates at the optimum temperature of 6 oC than offspring from all other fathers. However, the survival of offspring from father #1 at 8 oC was the same as the 6 oC optimum, while survival of offspring from all others was lower at 8 oC than at 6 oC. This suggests that offspring from father #1 are better able to tolerate a 2 degree increase in temperature. This interaction factor accounted for 20% of the variance in survival rates overall.
Warming oceans will negatively affect cod populations as embryos suffer decreased survival as temperatures rise, regardless of the father. This puts selection pressure on fish better adapted to survive in warmer temperatures. A comparison between western Atlantic cod fathers vs. Baltic cod fathers was not made in this paper due to the logistical constraints that prevented the researchers from rearing offspring from both types of father in the same temperatures. The Father-by-Temperature interaction suggests some provide genes that result in offspring that are less prone to mortality in warmer temperatures. These fish will have a selective advantage over others in the warmer oceans of the future. This study also identified that most mortality, and differences in mortality between offspring of different fathers, occurs at very early embryonic stages of development, flagging these very early stages as a critical bottleneck in cod development.
Ocean temperatures are expected to increase by 2-4 oC throughout the Atlantic cod range by the end of this century. That would put ocean waters at 8-10 oC, instead of the optimum 6 oC for these fish. Looking at the data from this experiment, the survival rate of fertilized embryos at 8 and 10 oC is much lower than 6 oC. This is bad news considering cod populations have been struggling to recover from overfishing for decades. Survival of offspring is the only way to replenish overfished cod stocks. Therefore, waiting to reduce fishing pressure until later when oceans are warmer may be very bad news for the prospect of a cod recovery.