It’s June. The streams are bursting from snowmelt and rainwater. Trees are budding. Birds are nesting. And in the river, something is moving – a mass of living bodies so dense that they fill the entire water from bank to bank, slipping quietly beneath the surface. This is the annual spawning migration of the Gaspereau.
Gaspereau, otherwise known as River Herring, are really two species of anadromous fishes – the Alewife and the Blueback Herring – that are common in rivers across eastern North America. They are anadromous because they spend most of their adult life at sea, but return to freshwater to spawn, usually to the same river where they themselves were born. Nova Scotia fishermen are quite familiar with Gaspereau, which serve both as a fishery in and of themselves, as well as bait for other species like Lobster or Striped Bass. Not highly prized at the table, they have sometimes been referred to as the Sunday fish, as in it would take you all day Sunday to pick out the bones! Nonetheless, Gaspereau play important ecological roles in both the marine and freshwater environments, and are ingrained in the culture of many coastal communities.
One of the primary rivers for Gaspereau in Nova Scotia is the aptly named Gaspereau River in Kings County, traditionally called Magapskegechk by the Miꞌkmaq people. Nestled amongst farmlands and lush vineyards, it flows from Gaspereau Lake into the Minas Basin – a tidal estuary at the head of the Bay of Fundy. The river has two hydroelectric generating stations, and fish have to climb up specially designed fish ladders in order to get around them. The riverbed habitat and lake form an important breeding ground for many species, including Rainbow Smelt, White Suckers and Atlantic Salmon, but the only Gaspereau seen here is the Alewife (aka the Kiack, Sawbelly, Kaspelaw or Abit Petŭbět’). The river also supports a long-standing tradition of recreational tubing, a staple summer activity in the Annapolis Valley!
As Alewife file into the fish ladder, I stand with my net at the ready. One swift sweep and up they come, scales glistening in the sunlight, into a tank I have already prepared for them. I am a marine biologist, and my job is to study what happens to these fish after they leave the river and return to the ocean. There is precious little we know about the marine lives of anadromous fishes, which really limits our understanding of the threats they face, and what we can do to protect them. One way to study this is by outfitting the fish with small tags, which send out an audio signal that can be picked up by receivers (essentially glorified underwater microphones). Each time a fish passes by one of these receivers, it leaves a record of its tag number along with a time stamp – a sort of “I was here” message. By putting out lots of these receivers we can see (or rather hear) where they go. Over the past few years, my lab has been working closely with local communities, the Offshore Energy Research Association (OERA) and the Fundy Ocean Research Center for Energy (FORCE) to tag different species in the Bay of Fundy, from Atlantic Sturgeon to Stripe Bass. When they swim off, we want to know where they go, especially if they go to areas where they might come into contact with people, or the things that we put into the water.
To the fish, this probably feels like an alien encounter! I pick them out of the tank one by one, put them to sleep, and they wake up with a stitch in their side and a tag inside of them. But I do my best to be gentle, and to handle them as little as possible. Once everyone’s tags are in, they get a few minutes to recover their senses, and then it’s back into the river! And just like that they are gone, but my work is only beginning. Over the next months I need to periodically check on the receivers in order to download the data and do basic maintenance like changing the batteries. Once I have all my detection files, it’s on to analysis. There are a lot of ways to look at this information – for example, I can draw maps to see where the fish have gone, or compare the timing of their migration with environmental factors like temperature or tides.
On this map, you can see the location of each receiver as a dot, with the size of the dot proportional to the number of fish that passed through it, and arrows indicating movements between receivers (thicker = more times fish moved this way). The FORCE site is where the turbines are slated to go.
Note that the lines represent general movement directions, not actual tracks! For one, most fish probably don’t swim in straight lines. An arrow just means that some number of fish were detected by receiver A, then receiver B, and so on for each pair of receivers. It’s useful for knowing how different areas of the study site are connected to each-other, but does not show what actually happens between the dots.
You probably know that the Bay of Fundy has the highest tides in the world! And with high tides also come fast currents. A small fish can easily get caught in these currents and be carried around the entire Minas Basin before it finally escapes into the lower Bay of Fundy, and from there to the open ocean. Unfortunately, I can’t track them that far, but even what I have is enough to make some conclusions. I can clearly see how the fish move back-and-fourth across the study area with the coming and going of the tide, especially once the spring tides come around at the start of July. Spring tides don’t actually happen only in spring! Instead the word refers to tides “springing forward”; spring tides happen just after a new or full moon, and they have the greatest difference between high and low water (i.e. this is when we get the highest high tides and lowest low tides). At this time the Alewife start to spread further and further into Minas Basin. It only takes 20 days after tagging for them reach Minas Passage, and in a few more weeks they are gone from the area entirely. My last detection is on August 14; what happens to my fish afterwards I can only guess. I hope that they reach their winter feeding grounds safely, and will be back again to spawn next year, but by that time their tag batteries will be dead and I will no longer be able to hear them.
You can see how the number of fish detected each day rapidly decreases over time. As fish spread throughout Minas Basin, there are less receivers to hear them and more place for them to go. Eventually the tag batteries can die, or fish can migrate outside of the monitoring zone. There is also the possibility of some fish being picked off by predators – Alewife are pretty low on the food chain, and a lot of things eat them!
Of course, we cannot know everything, even with such a neat technology. Some fish we never hear from, but does that mean that the fish died, that they lost their tag, or simply that they snuck past a receiver that didn’t hear them? This question is really difficult to answer, although we can make some educated guesses. It’s very hard to hear a tag when the currents are strong, like they are in the Minas Passage where the turbines are slotted to go. Imagine trying to hear a single beep in the middle of a crowded room – that is what listening for fish is like in the Bay of Fundy! So not every possible detection is heard, and I sometimes have to fill the gaps in my data like I’m playing connect the dots. Still, I can pick out the general trends. I can tell that tides are important in the migration out of Minas Basin, and also that quite a few of the Alewife (at least 40%) will get close to the turbine test site. We see this with some of the other species too. However, that does not necessarily mean that fish will hit the turbine. There was no operating turbine at the time of this research, and even if there had been, a detection only shows that a fish was present somewhere within a roughly 300 m-wide area during a fraction of a minute! To know more, we would a) need to have a turbine in the water at the time of tracking, and b) have receivers positioned around it so we can track the fish on a finer scale. Those are tasks for future research. But this study has shown two important things:
1) We can successfully use acoustic telemetry to monitor the migration on Alewife, 2) Alewife have the potential to overlap spatially with structures deployed in Minas Passage.
These findings highlight the importance of conducting monitoring in this area, and will help to guide future work, and that is how science can advance!
My research is only a drop in the ocean compared to the number of things we still don’t know about fish. And perhaps some things we never will. But the more we study, the more we realize just how fascinating and how sofishticated these animals are. Being a researcher is not always fun and games. Sometimes I am stuck outside all day in the wind and rain. Sometimes I have so much data that my computer crashes. It’s hard work, but I love it. And it’s a ways to escape my own world for a time and catch a glimpse of the fascinating lives of fishes!