Chasing Eddies!

As Leg 2 begins, we are extremely excited to begin sampling a mesoscale eddy located south of Cuba! Oceanographic features like these eddies can be very important habitats for larval fish, and we want to explore why and how this changes for different fish species.

NOAA Ship Tracker shows the Foster's unusual track line.
Scientists deployed XBTs en route in order to find the eddy.

There are four major ways mesoscale eddies can affect larval fish:

1. Larval distribution – baby fish can get caught in the eddy circulation (entrained) or be transported by eddies to other bodies of water.
2. Eddies also impact how much and what types of prey are available for the larvae to eat, which can in turn affect fish growth.
3. Larval fish can move up and down in the water column depending on the time of day (diel vertical migration), and this behavior may be impacted by eddy circulation patterns.
4. In addition to active transport, eddies also generate thermal variations in the environment that affect biological rates.

Left: one of many XBTs deployed on Leg 2
Right: Physical oceanographer Ryan Smith gets real-time
temperature data from an XBT.

On this leg of our survey, our station plan will be very different from our approach on Leg 1, as we'll be employing an "adaptive sampling" strategy. Usually we spend weeks before the cruise planning our sampling stations, however, mesoscale eddies are very dynamic oceanographic features, constantly changing and evolving, so we'll need to modify our sampling plan on the fly based on real-time analysis of the data we observe. In order to find the boundaries and "center" of our target eddy, we will consult daily satellite imagery and altimetry data, as well as our own in situ measurements of ocean currents and temperature. We'll also use hull-mounted and acoustic Doppler current profilers (ADCP) to determine the upper ocean current velocity, and temperature profiles from conductivity, temperature, depth (CTD) casts and expendable bathythermograph (XBTs) deployments to analyze the upper ocean thermal structure. All of these data together will help us to target the center of these circulation features, where we'll begin our sampling! Following this we will also sample along the edge of the eddy circulation (at its frontal boundary), and finally we will sample an area of common water outside of the eddy for comparison. 

While adaptive sampling can mean managing a lot of different types of data in real-time, it is extremely rewarding when the clues that the data provide direct you to the target area you are looking to sample!

Left: Scientist Cati Mena and Professor David Lindo prepare to deploy an XBT!
Right: David shoots an XBT off the stern - see it mid-air!

Plankton sample collection is only the first step! Once we get back on land, we’ll spend months sorting through samples, removing and identifying fish larvae and their prey. Then some of those fish larvae will be measured, their guts dissected (using VERY tiny tools!), and their otoliths (ear stones that have marks like tree rings!) analyzed to determine the growth and age of the fish. Finally, all of this information will be combined with the physical oceanographic data (which also has to analyzed) in order to answer our questions. It will be a lot of work but worth it when we get to present the final results and conclusions!

Images of the many steps of plankton processing: a. Unsorted plankton takes patience and a trained eye to find all the larval fish! How many can you spot? b. Once sorted, larvae must be identified - these are baby blackfin tuna. c. Individual fish can have their guts analyzed - check out how full this baby swordfish is! d. Otoliths can tell you how old a baby fish is, if you have a powerful enough microscope, that is. This otolith is less than 1mm in diameter!