Just in from the Ship
TC-99-07 Lobster Assessment
June 6-July 4, 1999
14, 1999 We have continued our lobster assessment
operations on the reef surrounding Necker Island. Every morning at 8:00 AM, we start
hauling lobster traps and measuring and recording the sex, species and size of the
lobsters caught. This data will be used to estimate the size of Necker's lobster
population and how much fishing the population can support. During most of the evenings,
we perform 40 mile ADCP transects to gather data on the ocean currents around Necker
Island reef. The planktonic lobster larvae are very dependent on these currents.
reproduce by laying eggs (right). The female lays her
eggs and attaches them to the pleopods (swimmerets) under her tail. Female pleopods are
larger than a male's and have hairs on them which assist in the attachment of eggs to
them. In Hawaii, egg laying occurs year around and its peak rate happens over the summer
months (June - August). Female lobster caught during this time often have thousands of
eggs attached under their tails. When first laid, the eggs are a bright orange/yellow
color. As the eggs near hatching they turn a brownish color.
The larvae (below, left) hatch out at about the size of
this "O". In order to grow, lobsters (and all crabs and shrimps) must shed their
hard exoskeletons. After the laborious job of wriggling out of its old exoskeleton, the
lobster immediately goes into hiding from predators. The lobster's new shell is very soft
at this stage and most animals will readily make a meal of it if possible. During this
time, the lobster's body absorbs sea water and swells (thus grows) and over the next few
hours the lobster's new shell hardens. Once hardened, the lobster remains that size until
the next molting.
lobsters look nothing like the adults as they are planktonic (free drifting) in nature.
Slipper lobsters are planktonic for the first 3 months of their lives, while spiny
lobsters are planktonic for the first 12 months or so. During the day, the larvae can be
found as far down as 80 meters (about 240 feet) and during the night they are usually with
in the 0-20 meter (0-60 feet) depths. They maintain this lifestyle for about 15 molts,
during which they eat other zooplankton (free floating animals) and phytoplankton (free
floating plants). The larvae are themselves food for larger organisms and a large
percentage of them are eaten. By the 15th molt (or so), those that survive are about 2-3
inches long, clear, with circular but flat (almost paper thin) bodies.They have small legs
and very short, almost minuscule tails.
this time, the larvae molt into what is termed the "transition" stage. During
this molt, the body actually folds upon itself, giving the animal height and reducing its
length and width. The larvae emerges from its former self looking like a small clear adult
and it's exoskeleton hardens into a protective shell. During the first stages of its life,
the young lobster simply drifted with the currents and used feathery paddle-like
appendages to maintain its preferred depth. In the transition stage, the juvenile becomes
an active swimmer and begins looking for a bottom of suitable (0-30 meters or 0-90 feet)
depth on which to live. Research in Australia suggests lobster juveniles can swim as much
as tens of miles in their search for bottom habitat. The same researchers also discovered
that juvenile lobsters usually swim directly for a reef or sea mount, possibly sensing
water borne chemicals (similar to smell) to determine the direction in which to swim. If
the animal is not successful within a certain time, it will sink beyond what it can
survive and perish. Thus the importance of ocean currents in bringing juvenile lobsters
near a suitable reef near the end of their planktonic stages.
reaching a suitable bottom, the juveniles begin a life of hiding under coral and rocks
during the day, and foraging for food at night. One of the main predators they must be
wary of is octopi, which love to eat lobsters. A lobster's main locomotion is by crawling
over the bottom. To escape predators they use the powerful muscles in their tails to
explosively propel them backwards in quick jerky movements.
Yesterdays cruise write-ups for TC-99-07