Trawling Through Time to Investigate Coastal Marine Ecosystems

March 29, 2017

A 10-day research mission to survey pelagic micronekton and plankton off the eastern coast of Oahu, Hawaii.

Cardinalfish

Cardinalfish (Photo: NOAA Fisheries/Jonathan Whitney).

Our scientists will spend the next ten days investigating the pelagic marine ecosystem and underlying oceanography of the leeward (east) coast of Oahu from the NOAA Ship Oscar Elton Sette

The project has two primary objectives related to the survey of pelagic micronekton and plankton off the coast of leeward Oahu. The first component is to perform a midwater trawl and bongo net survey from the Sette to compare with a series of midwater trawl and bongo net surveys from 1951–1978 off leeward Oahu, henceforth referred to as the baseline surveys (conducted by a mix of academic and government researchers). These baseline surveys documented species composition and abundance for four different components of the pelagic community, notably the mesopelagic fish assemblage, shorefish larvae, tuna larvae, as well as a suite of larger zooplankton taxa from two forage availability studies and an early trawl survey. Comparison of current composition and abundances to the baseline composition and abundances after 39–66 years will be a valuable scientific finding to ascertain how the pelagic ecosystem has changed, or not, over that extended time period. The sampling gears used in the baseline surveys from 1951–1978 are 10′ Isaacs-Kidd midwater trawl, Cobb trawl, and 70cm bongo nets.

Scientists conduct day and night bongo net surveys at various depths from 200 to 500 meters from the NOAA Ship Sette.

Scientists conduct day and night bongo net surveys at various depths from 200 to 500 meters from the NOAA Ship Sette (Photo: NOAA Fisheries).

The second component of the project is to establish a time series of micronekton and plankton abundance in the leeward Oahu area for ecosystem monitoring. Maintaining some measure of the health and productivity of the coastal ocean is extremely valuable, given scientific concerns about coastal fisheries, human impacts to ecosystems, and climate change. Some of the surveys from the historical comparison will be useful here as well as an expanded suite of sampling to cover additional components of the pelagic ecosystem, particularly the smaller zooplankton missed from the historical surveys. 

Why Leeward Oahu?

Leeward Oahu was chosen because this area was found to be a hotbed of scientific activity on micronekton and plankton in the 1950s to 1970s. It is also a highly-populated coastal area that is important to the local community.

What are plankton?

Plankton are the lifeblood of the ocean and the planet. They are a vital part of the food web at its very foundation and core. Plankton feeds the rest of the marine life in the ocean and what eventually provides seafood for our tables. Furthermore, most marine life starts as babies living in the plankton. Plankton can be microscopic, can live in very deep water far from land, and can be very rare. These three things make catching plankton very challenging. Scientists have to use special nets to catch plankton in a way that can tell us how abundant they are and filter enough water to catch the rare ones. As plankton become larger, they transition to a category of organisms called micronekton. Micronekton are difficult to capture for some of the same reasons as plankton, with the added challenge that they can swim away from nets.

Why are plankton important to us?

How can we tell if the ocean is healthy? Much in the same way a medical doctor would tell us if we are healthy or not. They take our pulse or perhaps order a blood test. Similarly, we can test or sample the ocean to see if it is healthy. Plankton are key to a healthy ocean. We can take simple measurements and sample the water to examine important properties within it. While ocean water has many important properties, clearly one of the most important properties relate to what is living in it, namely plankton.

Plankton collected from a bongo net trawl.

Plankton collected from a bongo net trawl is carefully counted (Photo: NOAA Fisheries).

What is "Trawling Through Time"?

We hope to compare ocean conditions around Oahu in the 1950s–1970s to the ocean today—to better understand ocean resiliency, ocean health, and ocean productivity through one of its most basic form of life: the plankton. We can use the same types of survey gear now in the same areas to see if the plankton are the same or different.

Why is this important?

The numbers and different species of plankton tell us something very powerful about the ocean. If the plankton is the same as it was decades ago, it helps us understand how stable the base of the food web is over time. If there are differences, we can investigate how plankton changed in response to long-term changes in the environment and better understand the health of the ocean.

Samples of small mesopelagic fish collected from a trawl

Samples of small mesopelagic fish (myctophidae) collected from a trawl (Photo: NOAA Fisheries).

Partners participating in this mission include scientists from the University of Hawaiʻi at Mānoa, American Museum of Natural History, San Francisco State University, and Pacific Islands Regional Office.