Common Merganser - Mergus merganser
There's a lot happening on the river! It is a source of food for many different species of wildlife. There are many aquatic insects, several crustaceans (some being microscopic), mollusks, fish, as well as reptiles and amphibians that inhabit the river. For some species, fish make up more than 90% of their diet. For other species, fish are just one of their food sources.
Common Mergansers are diving ducks that feed on mollusks, aquatic insects, crayfish, worms, frogs, small mammals, other birds, plants and fish! They mainly eat fish in the winter, when aquatic insects, frogs, worms, and crustaceans are less active.
Common Merganser - Mergus merganser
I saw this group of 9 Common Mergansers on the river one morning this week. It must have been a mother with adult-size juveniles! They all looked like they were females, but they probably weren't. After breeding, most ducks molt into "eclipse plumage" in which males acquire a female-like plumage. That's why in the summer, it seems as if all the male ducks have left! This plumage change also coincides with the molt of all flight feathers in the male ducks. The male ducks are quite vulnerable without their flight feathers, so the female-like plumage provides better camouflage than their breeding plumage. It takes about a month for the flight feathers to grow back! The males will molt again this fall, back into their breeding plumage. The females molt their flight feathers later than the males, usually after their young have fledged. The females molting process does not noticeably change their appearance. All juveniles look like females.
Common Merganser - Mergus merganser
All birds have a nictitating (blinking) membrane, a transparent extra eyelid. This serves as a diving mask that allows mergansers to keep their eyes open underwater, while they swim with webbed feet. When at the surface, they will swim forward with their head underwater searching for or chasing prey. In shallow water, they return to the surface to swallow their prey. In deeper water, they may swallow fish while still submerged.
Common Mergansers underwater - Mergus merganser
I was lucky enough to see the same group of Common Mergansers diving underwater this week! It was fascinating to watch! In the photo above, the large grey blobs with orange appendages are the underwater mergansers! WOW!!! Their heads are down, probing sediments and underwater stones for prey. Often when one bird dove in the group, the others followed the leader and dove!!! They can stay under for up to 2 minutes, but they normally dive for less than 30 seconds. So amazing!!!
Coastal Rainbow Trout Fry and Adult
Oncorhynchus mykiss irideus
Rainbow Trout!
The most common species of fish in the North Yuba River is the Coastal Rainbow Trout, which are native to California, but have been planted locally. There are also non-native German Brown Trout, that swim up to spawn from Bullard's Bar Reservoir. Occasionally you might also find a non-native Brook Trout that has flowed out from a higher elevation lake, during the high water of Spring.
Fish are preyed on during all stages of their life cycle, from eggs to adults. Their predators range from tiny dragonfly nymphs to large Black Bears!
Fish start as eggs which hatch into larvae. The larvae are not able to feed themselves, and carry a yolk-sac in their bellies which provides their nutrition. At this stage they are called "alevins". When they have developed to the point where they can feed themselves (mainly zooplankton), the fish are called "fry". When they develop scales and working fins they are called "fingerlings". This juvenile fingerling stage lasts until the fish is fully grown, sexually mature, and interacting with other adult fish.
Water Strider Nymphs and Adult - Gerris sp.
Aquatic Insects!
Water Striders start out as eggs which hatch into nymphs. The nymphs have 5 instars (periods of growth) between molts. In about 60 to 70 days the nymphs become adults. As adults they prey on spiders and insects that land on the water, as well as nymphs of their own species! Birds are the main predators of the adults. To avoid predation the adults can fly away or dive under water!
Water Striders are known for their curious ability to "walk-on-water"! They are able to do this for a variety of reasons. They use the natural surface tension of water, along with a water-repellent body covered in LOTS of fine hairs (up to several thousand hairs per mm), as well as long thin legs that distribute their body weight over a large area. The round shadows they cast, from the tips of their legs, are caused by the dimples their feet create on the surface of the water!
Stonefly Nymph and Adult - Plecoptera sp.
Stonefly nymphs will live underwater for 1 to 4 years, depending on the species. They prefer running rivers and streams, not lakes and ponds. They feed on algae, detritus, and plant materials under water. When their nymphal stages end, the nymphs will crawl out of the water onto rocks, their exoskeletons will dry out and crack open, the adult will pull itself out of the exoskeleton, pump up its wings, dry out, and fly away! As adults they feed on nectar, as well as algae and lichens. The adults usually live for about a month, just long enough to mate and lay eggs!
Caddisfly larvae in cases - Trichoptera sp.
Pictured above are Caddisfly larvae in their cases. These larvae can be found in streams, rivers, lakes, and ponds! The material used to make the cases varies between species. Inside its case, a Caddisfly larva makes a silken cocoon around its body. It will pupate in this silken cocoon before it becomes an adult. As larvae they feed on algae, as well as aquatic and terrestrial vegetation. As adults, Caddisflies feed on nectar! The larvae live for about a year, whereas the adults live for about one month! Thousands of these adults were clinging to our window screens at night, a few weeks ago! They are attracted to light, just like moths! One night I watched a Deer Mouse outside our lighted window, climb up the Hop vines and eat these insects one after another! WOW!
Dobsonfly Nymph - subfamily Corydalinae
Dobsonfly larvae live in the highly oxygenated (white water) parts of the river because they need lots of oxygen. They have 6 legs, all the other appendages are gills, through which oxygen is absorbed from the river water! They live underwater for up to 2 years, feeding on other aquatic insects, before they molt and fly away as an adult.
Whirligig Beetles - Western Aquatic Garter Snake
Gyrinus sp. - Thamnophis couchi
As adults Whirligig beetles spend most of their time on the surface of the water, looking for insects to eat. They also look for prey underwater. They can see above and below the water surface, because they have eyes that are divided in half horizontally!!! The bottom half sees underwater. The top half sees above water! If alarmed these beetles will dive underwater for safety. The middle and hind pairs of legs are "natatory" (adapted for swimming), and are quite hairy. Their front legs are used for grasping prey. Their outer wing covers are covered in a waxy layer that is water repellent! If their watery environment dries up, they can fly away to another area! After mating, the female lays eggs on the stems of submerged plants. The eggs hatch into aquatic larvae that live in streams. These larvae pupate in cocoons, above water, when mature. I find all of these adaptations to their watery environment astonishing!
Western Aquatic Garter Snakes are not poisonous, even though they might bite when agitated. They are active during the day, and can be mainly found in lakes, ponds, streams and rivers, or basking at the water's edge. They eat mainly fish, and amphibians and their larvae.
What's happening in the Lakes Basin?
Are the Tanagers, Orioles, and Grosbeaks all gone?
Check back next week for the answers to these questions and more!
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