by David Grant
Over a three-day period in June,
1986, I noticed a curious discoloring of the waters associated
with the outgoing tide at the Horseshoe Cove marsh on Sandy Hook,
a sand spit extending into Raritan Bay at the northern end of
New Jersey's shore.
The ten-acre marsh is dominated almost entirely by high-vigor cordgrass (Spartina alterniflora). There is only one outlet for water exchange with Sandy Hook Bay, except during severe storms when overwash from the adjacent beaches occurs. The culvert pipes that channel the tidal creek waters are an excellent site to observe fishes and other creatures as they move between the marsh and the bay. The culverts form a sill that maintains the water level in the marsh high enough to prevent the tidal creek from drying out at low tide. Horseshoe Cove, so named because of the hordes of horseshoe crabs that nest there each year, is a nursery for many other estuarine creatures like crabs, mullet, snappers, killifishes, and shiners.
I noticed a milkiness of the water, most pronounced on June 6. Discolored water is not unheard of around Sandy Hook, where over the years the Army and the residents, as well as boaters and the general public, have been responsible for accidentally or intentionally filling marshes, shoring-up eroded points, or dumping materials. Plankton blooms are frequent and intense.
This milkiness was new to me, and upon closer investigation it was apparent that the gyre formed by the outgoing rush of marsh water delineated the boundaries of the patch. To my surprise, the water on both sides of the culvert pipes was crowded with thousands of Atlantic silversides (Menidia menidia) (AKA shiners, spearing) struggling unsuccessfully to enter the narrowing entrance to the marsh against the ebbing tide current. Those fish too weak to overcome the flow or arriving too late to cross the shallow sill were releasing their milt and eggs spontaneously, in the excurrent gyre, among the stems of cordgrass and on the sandbar. Many were stranding. Concentrations of spawning fishes are not rare in the sea, in fact, "whitening of the water with milt" is reported in the literature on shiners and herring. But the behavior of this group was odd. Most were skimming the water's surface, mouths agape, gulping air as though they were stressed.
I took water samples and collected a bucket full of shiners. (By now other fishermen had noticed the windfall of stranded bait and competition for the fish suddenly became fierce.) The results are interesting. Outgoing marsh water was 20º C, pH 8.0 and about 85% saturated with 8.0 ppm of dissolved oxygen, typical for that time of year. Adjacent milky "trapped" water in the gyre was the same temperature but less alkaline (pH 7.6). Dissolved oxygen levels were below 1.0 ppm (the lowest reliable point for the testing kit I have), less than 15% of saturation and dangerously low for most marine life. The presence of organic material can affect chemical tests for dissolved oxygen, but these low readings and the stressed behavior of the fish struggling to remain crowded at the entrance to the creek, seem to indicate that their concentration within the confines of the gyre, with attendant respiration, and their release of milt and eggs was sufficient to alter the local chemistry of the water. Of the 30 stranded fishes I recovered, most (76.6 %) were females - on the average they were 10 percent longer and one-third heavier than the males. All were gravid and some had different sized eggs, indicating an extended spawning season.
The eggs are beautiful, yellowish
gems that adhere to each other and vegetation with thin elastic
filaments from the chorion. When I placed them in our aquarium
they were eagerly taken by killifishes, crabs, and lobsters,
but ignored by spot, ling, and bass. Examination of scales of
the largest shiner specimens (10-12 cm) indicated that these
were all a year old or less. The absence of any new growth on
the perimeter of the scales implies that most of their energy
in the spring is devoted to producing spawning products. Their
behavior shows that at times reproductive instincts can overwhelm
other survival instincts, and although not as dramatic as a run
of salmon, the ubiquitous shiner, like all fishes, is nonetheless
fascinating to observe.
Perhaps in the spring we should be, gathering to witness "arrival" of this determined and important baitfish as we do with the venerable horseshoe crab.
Something to think about:
- What advantages would there be to the females of a species being larger?
- What is one way to determine the age of a fish?
- What features can affect the pH and dissolved oxygen content of water?
- Why are Latin names important to know and include when writing, discussing or studying different organisms?