Article: Berejikian BA, Moore ME, Jeffries SJ (2016) Predator-prey interactions between harbor seals and migrating steelhead trout smolts revealed by acoustic telemetry. Mar Ecol Prog Ser 543:21-35 doi:10.3354/meps11579
Helping to conserve fish populations can be tricky. Fish are constantly moving, their habitat is changing (due to environmental and human factors), and, in consequence, the composition of the food web is continually restructured. This makes it challenging to identify exactly what factors are influencing depleting or growing fish populations.
Understanding predator-prey dynamics can show us the cause of decline in certain fish species. Some conventional approaches to looking at the food web and understanding the predator-prey interactions are using a combination of monitoring diet composition by looking at gut content and foraging behavior using satellite tags. While these tags are great for tracking and following predators, they might not be so great for prey species. The tags used send off a small hum which harbor seals notice. Lab studies have shown that harbor seals can associate the sound from the tags with food. This raises the question of whether these tags are acting as a dinner bell for harbor seals.
Steelhead trout migrate directly through Puget Sound (home of the harbor seal) en route to the Pacific Ocean as smolts (small trout who are fresh from freshwater streams) and return as adults to spawn in their natal streams. Berejikian et al. set off for Puget Sound to investigate the predator-prey interactions between harbor seals and steelhead trout smolts, testing the potential effect an audible tag has on survival.
Over 200 smolts were outfitted with a tag which emitted an acoustical signal every 30-90 seconds, on random. Only half the smolts that traveled through the harbor seals home (Puget Sound) were implanted with tags that emitted an acoustical signal (Fig. 1). The other half had tags that stayed silent for the first leg of their journey. The tag was programmed to turn on and emit a signal ten days after release, approximately the amount of time it would take them to travel to the outer banks and out of the harbor seals home area. This would indicate whether the harbor seals were more attracted to smolts with an acoustical signal or not.
Twelve adult harbor seals in Puget Sound were outfitted with backpacks. These backpacks allowed the researchers to track the harbors seals path and see how deep they were diving. The backpacks also detected the signal given off by the smolts tag, which allowed the researchers to see if the harbor seals would be within range to distinguish their “dinner bell” prey (tagged smolts) from their normal prey (untagged smolts and other fishes).
So, are tags really dinner bells for harbor seals?
An estimated 226 of the 243 tagged smolts survived their migration through Puget Sound. Whew! This does not mean they went unnoticed, however. Based on the information retrieved from the 11 seal backpacks (1 was lost), seals were detecting 44 different steelhead smolt signals. Half of these were detected after the smolt outmigration period (Fig 2). This means the seals detected the tagged smolts, but chose not to hunt them down.
Unfortunately (for the smolts, at least) nine of the steelhead trout smolts were consumed by harbor seals, based on the signaling pattern of tags and their final locations. While this does not statistically prove that the smolt tags were acting as a dinner bell for the harbor seals, a larger scale study needs to be done to better understand what is occurring and why the harbor seals aren’t always going for the dinner bell. Previous research has shown that harbor seals primarily feed on herring and cod, but this study is showing just how easy it is for the seals to consume smolts (they did consume 9!). With dwindling stocks of cod and herring, tagged fishes are next in line for the harbor seal’s dinner. When further research studies are examining conservation strategies for fish species, acoustic tags need to be used with caution. They might just be disrupting the food web dynamics.