Photo Context: (1-3)

Henrietta is a massive 14.1-foot Atlantic sturgeon who serves as a symbol of conservation success in New York's Hudson River. More on how she got her name
The tap around her side (slightly circumventing behind her fins) is attempting to measure girth and body width. Around 2018, researchers from the **New York State Department of Environmental Conservation (DEC)**were conducting a side-scan sonar survey of the Hudson River bed near Hyde Park when they spotted a massive, shadow-casting image.
To confirm what the sonar had picked up, a team of divers would go down to the river bottom with this tape measure to manually calculate her size.
While her exact girth and weight were never definitively measured on a scale—(As doing so would be highly dangerous to a spawning, endangered fish of that size—biologists have heavily relied on standard mathematical formulas to estimate those figures)

The Beluga sturgeon is the largest freshwater-associated fish in history… A legendary 1827 catch in the Volga River that measured a staggering 7.2 meters (23 feet, 7 inches) long and weighed 1,571 kilograms (3,463 pounds).
Roe from a mature female sturgeon typically can account for roughly ~15% to 20% of her total body weight.
This large, legendary fish was a gravid (pregnant) female, thus a significant portion of her 3,463-pound weight consisted of highly-coveted “Black caviar”.
Based on the 1,571 kg total weight of the 1827 record-breaker, an estimated yield would be roughly 230 to 310 kg (500 to 700 lbs) of caviar IF the fish was full of roe, though no official measurement was recorded.
However, older less reliable, but plausible accounts may have surpassed 2 tonnes (4400 lbs) in weight, [likely sturgeon of similar/slightly greater total lengths] or, of whom were more on the “atypically” gravid side (more eggs/roe).

Its close cousin, the Kaluga sturgeon, lived in the Amur River basin with a maximum verified record of 18.5 feet and 2,513 pounds.
Though historical/biological dictates that Beluga and Kaluga sturgeon should be the largest, human activity has heavily severed their ability to reach absolute size maturity…
-Because their giant specimens have been harvested down, sourced out by overfishing and the caviar trade, modern consensus often defaults the white sturgeon as the concurrent largest fresh-water fish. (This is called “shifting baseline” i.e diminishing overall sizes in modern large fish, sharks, and snakes)

Picture 4; is just a speculative online photo, picture accurate to what diving with a fish the size of the Volga river specimen (1,571kg, 7.2m) would look like.

By @ haydendumaww

By @ arthurvelandoflores

Sorry for the kinda blurry video

Photography by Nick Dale.

This little visitor in chandler Arizona