During late November and early December, 2015, Auke Bay harbor put on a wildlife spectacular, drawing photographers, reporters, and just plain gawkers (such as me). Hordes of young-of-the-year herring, mixed with a few capelin and sand lance, milled around the docks and boats. The banquet of small fish also drew many predators, who put on a good show for observers.
Why are there so many young herring in the harbor this year? There are possibly several reasons, suggests Michelle Ridgway (Oceanus Alaska). It may have been a good spawning season in spring. The sunny spring, plus an El Niño, warmed the harbor waters, even at considerable depths, and all the spring run-off from the soggy land brought in nutrients. Those conditions produced a fine bloom of phytoplankton, which led to good body condition and burgeoning populations of zooplankton. For example, Ridgway has noticed extended reproduction of little shrimp-like crustaceans called mysids, extra-large fat globules in copepods, an abundance of amphipods not far from the surface, and an unusual influx of ‘sea butterflies’ (molluscs that fly through the water). That made excellent foraging for baby herring. In addition, young herring may seek protection from the massive maws of humpback whales by moving into shallower bays and harbors, with docks and boats, where the whales are less likely to forage intensively.
Herring and other so-called forage fishes often form densely packed balls, especially when predators are lurking about. When a predator dives through the ball of fish, the survivors scatter in all directions, but not far and only briefly, before returning to the tight cluster. Researches have called this behavior ‘the geometry of the selfish herd’: each fish trying to put as many other little fish as possible between itself and predators. The result is a tight ball of nervous, jittery fish.
The baby herring in the harbor had every reason to be jittery. The millions of small fish were being attacked on all sides by throngs of predators. They may have eluded most of the whales in the confines of the harbor, but other predators took advantage of the great aggregation.
A gang of Steller’s sea lions cruised rapidly back and forth, diving continually, probably after pollock that were gorging on the herring. The pollock drove the little fish toward the surface. The sea gulls knew this, of course, and hung about, just waiting for the fleeing fish to get close to the surface where the gulls could nab them. Indeed, the fish were caught ‘between the devil and the deep’ (the gulls and the aquatic predators respectively).
Several harbor seals were there, some with well-grown pups. They did their share of fish-driving too, but usually not near the sea lions. I watched one seal surface-swimming slowly along, on its back, in a most relaxed fashion. It may have been looking down into the depths, for eventually its head went down, followed by the plump body, into a mob of fish.
Scattered Pacific loons and little clusters of common mergansers foraged away from the biggest crowds of predators. Marbled murrelets in snazzy winter plumage could be observed at close range; they were much less skittish than in the breeding season.
The most amazing sight was the huge flock of common murres—many hundreds of them. They rafted up just beyond the last float and split off occasional smaller bunches that moved in among the inner floats. They, and everybody else except the gulls, avoided the sea lions that charged to and fro. I had never seen so many murres before, except at the St Lazaria nesting colony on the outer coast. The murres talked to each other constantly, except when they were diving.
That huge concentration of murres was arguably the most unusual happening in the harbor. All the other predators visit the area rather regularly to feast on small fish that spend the winter there. Although murres nest on the outer coast, they tend to move closer to shore in winter, congregating where prey is abundant. But we don’t customarily see the murres in such numbers in Auke Bay harbor. Furthermore, over near Glacier Bay, good observers reported uncountably huge numbers of murres moving about.
The throngs of murres may be a sign of bad news, however, according to John Moran (NOAA). A mysterious oceanic anomaly in the Gulf of Alaska called the Warm Blob (because water temperatures are as much as five degrees (F) above average) created nutrient-poor conditions that greatly reduced productivity and thus decreased the abundance of fishes that feed on plankton, or at least caused them to move to deeper waters where diving birds can’t get them. The Blob developed in 2013 and its effects have contributed to reduced nesting success and great mortality of some marine birds in the Gulf. The poor food supply may have been one factor that drove the murres we’ve seen in Auke Bay out of their usual foraging areas in search of better feeding conditions.
All those baby herring in the harbor seem to offer a ready banquet but, in fact, those little fellows have very little fat because they put their energy into growing as fast as they can. On a gram-for-gram basis, they are much less rewarding than capelin or adult herring, for example, and even less than krill, according to data of Moran and colleagues. So a murre or any other predator would need a lot of them in order to survive—and that’s certainly what was available in the harbor.
Humpback whales were reported to pass by the harbor upon occasion, but there are other spots where they might find better foraging. Seymour Canal is a good place for foraging on krill, for example, and adult herring (far more nutritious than the young ones) from all over Lynn Canal winter in deep, dense schools northward of Tee Harbor. When a whale dives deep through such a school of herring, some of the fish try to escape up shallower water, but there the sea lions can get them. Sea lions themselves may attract the attention of transient killer whales; a few years ago, Moran watched killer whales take down five sea lions (plus two probable kills) in five days. But in Auke Bay harbor, the foraging sea lions were quite safe from the killers.
Eagles were notably scarce in Auke Bay harbor during this extravaganza, although they are known to feed on murres (we saw the evidence in Berners Bay one spring). Perhaps the eagles sought out the adult herring to the north.
A little squad of goldeneye ducks quietly kept to themselves along a rocky shore of the harbor. Seemingly uninterested in the shimmering mass of herring, they may have been looking for molluscs.
Thanks to John Moran of the NOAA lab and Michelle Ridgway of Oceanus Alaska for extensive discussion, not all of which could be packed into this essay.