Recent Studies of Sportfish in the Navesink River

April 28, 2017
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Sportfishermen on the Oceanic Bridge near the mouth of the Navesink River waiting for a school of bluefish to swim in close. Photo by By Clyde L. MacKenzie Jr.

By Clyde L. MacKenzie Jr. |

During May and early June, local sportsfishermen can catch bluefish, striped bass and weakfish in the Navesink River, but the fish are not present for long stretches of time. Research by three scientific teams of the J. J. Howard Marine Research Laboratory, NOAA Fisheries Service on Sandy Hook, has shown when and why these fish occur at certain times and revealed some facts about their biology.

One group inserted electronic tags into the muscle of some fish of each species and was able to follow their individual movements. Environmental conditions – mainly prevailing temperatures and available foods – broadly control the fish residence times. At its westerly end, near Coopers Bridge, Red Bank to Middletown, the 5-mile long Navesink River encounters freshwater coming from the Swimming River.  The freshwater keeps these fish from going more westerly and into it. The fish may use the river for days or a few weeks and they may switch to the Shrewsbury River or Raritan Bay, as they seek optimum conditions for their survival and growth.

The sportfish enter the Navesink River from the Shrewsbury River and before that from Raritan Bay. Bluefish come into the Navesink River in May when their principal prey, the menhaden – or bunkers – swim in with them. The bluefish remained in the river for three to four weeks in past years, feeding rapidly and having a large impact on the numbers of their prey.  They left the river when temperatures rose above the mid 70s in early summer.  The small snapper bluefish, about 6 inches long, originate from the spawnings of adults over the continental shelf as they swim northward in the spring.  They appear in the river in June, and they can remain for about three months until October when temperatures fall into the 60s.  The snappers eat exclusively young-of-the-year menhaden.

The largest striped bass enter the river in April and May.  They remained for one to two weeks in past years. An early egress of the large bass coincided with the departure of their food, the large adult menhaden. Many smaller bass had longer residences. Temperature had a great effect on the striped bass, and they left the river rapidly when temperatures warmed. Several tagged bass made two or three excursions from the river.  Three bass that left the river in June and July returned in August or September after absences of more than 50 days.

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Weakfish also entered the river in May and some remained for more than two months. The biologists caught the weakfish by using gillnets in May, when they fed on invertebrates including shrimp, lady crabs and clam worms. By early July, the weakfish began feeding on fish and could be caught for tagging by hook-and-line. They remained during some of the warmest temperatures but left when the temperature exceeded the low 80s in late July and early August, and they left permanently in October when temperatures became too cool.

The biologists learned what these fish eat and how they reproduce.  The prey includes small fishes, such as menhaden and silversides, and invertebrates –mysids (small crustaceans related to shrimps), shrimps and even small blue crabs.  The age-0 menhaden are 2 to 6 inches long and are eaten by bluefish and striped bass during May, but the adult menhaden migrate out of the river in June and July and then return and are present in large numbers through October.  Sometimes, blue crabs were the dominant prey of bluefish. The weakfish eat shrimp and mysids in May and June, but later eat menhaden and silversides.

Menhaden feed mainly on copepods (small swimming crustaceans that are 1-2 mm long), and also fish eggs, the swimming larvae of mollusks and worms, and diatoms, all of which can be present in great numbers in the Navesink River. The menhaden’s foods cannot be visually identified by examining their stomachs which are gizzard-like and grind food into an amorphous paste. The silversides eat pretty much the same foods as the menhaden.  Constantly swimming rapidly and having a high metabolic rate, silversides need to eat about four times their body weight each day to achieve maximum growth. A food intake only about equal to their body weight is a minimum diet for them.

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Silversides reproduce in May through July. They lay eggs along the upper edges of the river’s high banks at distances six to eight feet above mean low water. The eggs remain exposed to air for about 10 hours between the two daily tidal cycles.

The third group of Howard Laboratory biologists found that large numbers of shrimp are in the river’s coastal shallows at low tide. The shrimp move into the intertidal zones with incoming tides to feed. One type of their prey is the eggs that killifish and silversides have laid on the surfaces of grasses and stems of erect plants, such as Spartina.  The placement of these eggs away from full-time susceptibility to shrimp and other predators gives the two fish species a better chance for survival.

Most of the fish in the river spend their winters in more southern waters. In November, bluefish leave for the south and they concentrate along the outer edge of the continental shelf. Weakfish spend winters in the offshore waters of Virginia and the Carolinas. Most of the schools of adult menhaden have moved to waters off the North Carolina coast by December.  These fish are followed later by large numbers of juvenile menhaden that have emigrated from northern rivers and bays. Menhaden spawning occurs year-round and can even peak off the coast of North Carolina during winter. The silversides occupy the offshore of the continental shelf in winter. An exception is the striped bass that usually remain in northern rivers, such as the Hudson, several rivers in southern New England and in the Chesapeake Bay.

Clyde L. MacKenzie Jr. is a biologist at the James J. Howard Laboratory, NOAA Fisheries, on Sandy Hook. Over the past 50 years, he has authored many scientific and popular articles about shellfish and finfish.  For more information regarding the work described above, contact Clyde at clyde.mackenzie@noaa.gov, or Dr. Beth Phelan at beth.phelan@noaa.gov.


This article was originally published in the April 27, 2017 print edition of The Two River Times

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