Sneaker Salmonids

let's look at the phenomenon in salmon, trout and charr commonly known as "jacks". Scientists call these male salmonids "sneakers". Anglers frequently catch these fish and it is interesting to consider why this occurs to some males in a given lake or sea-run salmonid population. One thing that people immediately recognize is that these fish appear to be fresh fish with silver colouration and very minute male characteristics, such as a kype (or hook jaw). In fact, these indicators often confuse things because "jacks" remain silver throughout the spawning run. It is a remarkable male reproductive strategy that is almost entirely opposite to the other more common male strategy that people are much more familiar with. To look at this more specifically, let's look at the typical scenario for spawning sea/lake-run salmon, trout and charr.

Salmonids build nests, properly termed redds. It is the female that fans the redd with her tail to dig a depression for her eggs. Males compete with one another in hopes of winning the privilege of fertilizing her eggs. As one might expect, this contest can get quite violent and it's no surprise that spawning males have evolved into large fierce looking fish with brilliant colouration. They have big kypes and hump backs. The bigger and they are, the more apt they are to win jousting contests with other males and the brighter they are, the sexier they are to females. Once the female chooses a dominant male, the buck must defend his territory from the hordes of males trying to spawn. So where does the "jack" fit in? If the trend for large size, big kype and bright colours holds true, then jacks should be very unsuccessful in spawning.

For simplicity, if you consider that all males have one unit of energy to invest into spawning, the larger males use most of this energy in making themselves attractive to females and formidable opponents to other males. Another major investment comes in the form of energy expended to defend a territory from other would-be suitors once chosen by the female. All of this comes at a cost and the trade-off is that large males produce fewer sperm.

Jacks, on the other hand, do not invest in large size; hump backs, bright colours and big kypes. Instead, they put nearly all of their energy into sperm. Jacks produce proportionally more sperm than larger males. With an abundance of sperm, jacks can afford to waste some whereas the larger males have to make theirs count. Continuing with the cost-benefit argument, large males are typically older than jacks and there is a cost associated with waiting to spawn. With potentially negative impacts coming from pollution, predators, diseases and parasites (among other things), the longer they wait to spawn the more likely it is that something can go wrong. Of course if they reach a large size they will be more likely to be chosen by females because they possess genes that have allowed them to survive, so the payoff can be huge. On the other hand, jacks typically mature at a younger age and thus have less risk associated with maturing.

The discussion thus far has still not accounted for the dominant behaviour that the larger male exhibits. If the smaller male wants to be successful, he has to get past the male guarding his territory. Remember that scientists call this male strategy "sneaking", so this offers an answer as to how they pull it off. One scenario involves jacks being found in numbers. One sneaker male will charge into the redd where he is immediately chased off by the large male. During the commotion, a second jack sneaks in and deposits his sperm before he too is chased out of the redd. Often this attempt at fertilization comes while he is being violently escorted out of the redd (or he fertilizes on the fly so to speak). This behaviour also coincides with non-spawning salmonids and members of other species trying to get a free meal of eggs. In the end the jacks fertilize a portion of the female's eggs. While not as successful as the larger male strategy, sneakers are nonetheless maintained in nearly all salmonids.

The next obvious question is what causes a male to become a sneaker? Is it genetic or is there something in the environment that causes a male to become a jack? It is almost certain that there is a genetic contribution and several genes in association with one another control this. Sneakers, it appears, have male offspring that become sneakers. Much of the understanding of the genetics comes from research directed towards aquaculture. Jacks are bad for business because they mature before reaching market size and as most anglers know, spawners don't taste very good. One interesting thing is that farmers remove early maturing fish from their brood stock and yet they keep showing up in subsequent generations. It is obviously more complicated than sneaker males producing more sneakers in the subsequent generation.

To add to the story, in field studies suggest that sneakers appear in certain environmental conditions, specifically low water. In the lowest water years (i.e. years with very little rain or snow melt) the proportion of sneakers increases. This is very strange since the process leading up to these fish being mature is quite long and time consuming. If the environment dictates the male reproductive strategy, then how do salmonids know if it's going to be a low water season? It's a very interesting idea that nobody really has an answer to. Regardless of which cues bring on early male maturation, it is clear that both genetics and the environment play a significant role in producing sneaker males.

The sneaking or jack phenomenon is an amazing male reproductive strategy. It is one of countless examples that make fishes fascinating. Next time you come across spawning salmonids in your favorite river, sit back and watch what goes on. It can make for some exciting experiences that don't involve your fishing rod. However, if you want to observe spawning behaviour, watch from a distance. Please don't disturb spawning salmonids, as they are doing their best to ensure more fish for the future.