Sunday, June 11, 2017

What Are They Eating?

Watching Richmond and Rosie and their chicks, Rivet and Whirly (really?) has become a real obsession for me.  The two cameras near their nest site make it possible to get up close and personal with the osprey family. 

I have seen Richmond - the male - bring in some sizeable fish.  As near as I can tell, they are all Striped Bass.  


Here's a screengrab from the nest-cam of Richmond arriving with a hefty striper. A feast ensues...

The chicks seem to thrive on the stripers, and their wings are becoming truly prodigious. 

Richmond is well-equipped for hunting.  Just look at those talons!

As for his quarry...


The striped bass, Morone saxatilis, was first introduced to the San Francisco Bay system in 1879 when 132 juveniles, taken from a small New Jersey estuary, were shipped across country by rail and released into Carquinez Strait (Skinner, 1962). A second plant of 300 fish was made in 1882 in lower Suisun Bay. Conditions in the Sacramento-San Joaquin estuary were obviously ideal, for the species flourished beyond all expectations and now supports a valuable recreational fishery within the Bay.

Today the bulk of west coast striped bass production occurs in the San Francisco Bay estuary. The species is anadromous, migrating in winter and spring to the Delta and upstream to spawn. Information on the biology of the striped bass, population size, and environmental factors affecting the species is presented by Stevens (1979).
There were originally no striped bass in California. They were introduced from the East Coast, where they are found from the Gulf of St. Lawrence to Alabama. The initial introduction took place in 1879, when 132 small bass were brought successfully to California by rail from the Navesink River in New Jersey and released near Martinez. Fish from this lot were caught within a year near Sausalito, Alameda, and Monterey, and others were caught occasionally at scattered places for several years afterwards. There was much concern by the Fish and Game Commission that such a small number of bass might fail to establish the species, so a second introduction of about 300 stripers was made in lower Suisun Bay in 1882.

In a few years, striped bass were being caught in California in large numbers. By 1889, a decade after the first lot of eastern fish had been released, bass were being sold in San Francisco markets. In another 10 years, the commercial net catch alone was averaging well over a million pounds a year. In 1935, however, all commercial fishing for striped bass was stopped in the belief that this would enhance the sport fishery. 

Biology (Morone saxatilis)


Striped bass begin spawning in the spring when the water temperature reaches 60 degrees. Most spawning occurs between 61 and 69 degrees and the spawning period usually extends from April to mid-June. Stripers spawn in open fresh water where the current is moderate to swift. The Delta, especially the San Joaquin River between the Antioch Bridge and the mouth of Middle River, and other channels in this area, is an important spawning ground. Another important spawning area is the Sacramento River between Sacramento and Princeton. About one-half to two-thirds of the eggs are spawned in the Sacramento River and the remainder in the Delta. Female striped bass usually spawn for the first time in their fourth or fifth year, when they are 21 to 25 inches long. Some males mature when they are 2 years old and only about 11 inches long. Most males are mature at age 3 and nearly all females at age 5.

Stripers are very prolific. A 5-pound female may spawn 180,000 eggs in one season and a 15-pound fish is capable of producing over a million eggs. This great reproductive potential and favorable environmental conditions allowed striped bass to establish a large population within a few years after their introduction in California. Striped bass often spawn in large schools. On one occasion, CDFW biologists observed a school of several thousand bass at the surface along the bank of the Sacramento River above Knights Landing. Small groups of three to six bass frequently segregated from this school and splashed and churned in the main current of the river in the act of spawning. At times, five or more groups of bass were observed spawning at once. Usually, a large female was accompanied by several smaller males. While the eggs are still in the female, they are only about 1/25 inch in diameter, but after release, they absorb water and increase to about 1/8 inch in diameter. The eggs are then transparent, making them virtually invisible. During the spawning act, eggs and milt are released into the water. The milt contains microscopic sperm cells which penetrate the eggs and cause them to develop. Striped bass eggs are slightly heavier than water, so a moderate current is needed to suspend them while they develop. Without adequate water movement, they sink to the bottom and die.

The eggs hatch in about two days, although the length of time may be somewhat shorter or longer depending upon temperature; hatching is quickest in warm water. The newly-hatched bass continue their development while being carried along by water currents. At first, the larval bass are forced to subsist on their yolk, but in about a week they start feeding on tiny crustaceans which are just visible to the naked eye. By August, they are about two inches long and are feeding primarily on mysid shrimp and amphipods, both bottom- dwelling crustaceans. At this time, they are most numerous from the western Delta to Suisun Bay.


The age of striped bass is recorded on the scales by a series of growth marks. The winter is a period of slow growth, during which a series of closely spaced rings form around the edge of each scale. The age of an individual bass can be determined by examining a scale under a microscope and counting the number of such closely spaced bands of rings, called annuli. Examination of many thousands of scales has provided a basis for determining the rate at which striped bass grow. On average, bass are four to five inches long at the end of the first year, 11 inches at the end of the second, 16 inches at the end of the third, and 20 inches at the end of the fourth year. A striped bass that is 36 inches long normally is about 12 years old. A bass 48 inches long, and weighing over 50 pounds, is over 20 years old. You can estimate the age of an individual striped bass if the length or the weight is known. The largest striped bass on record weighed 125 pounds and was caught in a seine net in North Carolina in 1891. Another very large one, weighing 112 pounds, was caught in Massachusetts many years ago. No stripers over 100 pounds has been caught on the Pacific Coast. There is an authentic record of a 78-pound bass from a San Francisco fish market in 1910. The current California sport record for striped bass is a 67-1/2-pound fish caught in O'Neill Forebay, Merced County, in May 1992. 

The current world record for a landlock Striper was caught on Feb 28, 2013 by James Bramlett on the Black Warrior River in Alabama weighing 70lbs (45 inches long with a girth of 37.75 lbs) source


Striped bass are voracious feeders. They generally feed on the most available and abundant invertebrates and forage fish of the appropriate size.

Initially, small bass feed on tiny crustacean plankton, but, after a few weeks, the favorite food becomes the mysid shrimp and amphipods. Mysid shrimp are most numerous where salt levels are 1–20 percent of sea water. Young striped bass are most numerous in the same area. Larger stripers tend to prefer larger food items. In San Francisco Bay, anchovies, shiner perch, and herring are important in the diet. Anchovies, sculpins (bullheads), and shrimp make up the bulk of the diet in San Pablo Bay. In the Delta and upriver areas, larger bass feed mainly on threadfin shad, young striped bass, and other small fish.

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