header image

Archive for Leif Rasmuson

Spring Transition

Posted by: | June 6, 2012 | 2 Comments |

This last term I took Comparative Embryology and Larval Biology taught by Dr. Maslakova at OIMB.  In the course we had the rare opportunity to witness first hand the development of many phyla of marine invertebrates.  We also maintained our own cultures of larvae changing their water and feeding them every other day.  The experience of  watching the organisms develop right before our very eyes was one I will never forget.  The class was very informative and I learned a lot about the development of many species.  One of the assignments in the class was to photograph the larva or embryo of an organism and write a brief blurb for the course blog.  I posted about crab zoea, barnacle nauplii and a polychaete called Polygordius.  I highly recommend that folks check out this blog and get a feel for what the larvae of many organisms look like.

 

IMG_3992

In my advisors previous research he found a correlation between the date of the year of the spring transition  and the amount of megalopae returning.  Our hypothesis is that upwelling that occurs following the spring transition brings the megalopae onto the shelf. My goal was to conduct a research cruise the week before the spring transition and a cruise the week after the spring transition to see if the number of megalopae on the shelf increased. Surface plankton samples and CTD casts were conducted on a 10 km transect perpendicular to shore.  We used a manta net (pictured to the left) to collect surface plankton tows.  The net works by skimming across the surface of the water collecting a sample just from the top fraction of water.

IMG_4084We conducted the tows at night and in the surface because previous research has found that the megalopae of Dungeness crabs inhabit the surface of the water column at night.  After deploying the net we towed it behind the boat for 10 minutes (picture to the right).  The cruise before the spring transition was rough and we had wind gusts to 45 knots. The entire crew did great and nobody got sea sick!!! The weather during the cruise the week after the spring transition was much calmer and was rather enjoyable.  We caught many interesting organisms such as salps and doliolids. The night we were out there was a great sunset and we all had the opportunity to watch it.  I am still in the process of analyzing the samples but will post again with the results later.

IMGP0017On April 2nd I put out our light trap to monitor recruitment of megalopae and I have been checking it daily. The photo to the left is of a sample collected on May 28th.  In the photo are approximately 60,000 megalopae that had climbed into the trap during the course of the night.  The recruitment events are highly pulsed and catch has ranged from 60,000 animals in a night to 0 animals in a night.

under: Leif Rasmuson, Robert E. Malouf Marine Studies Scholar

Orals are done!!!

Posted by: | February 17, 2012 | 1 Comment |

So last Friday I had my oral exams and I passed!!  Although the last few months of studying have been very stressful, the time I put in was beneficial.   The whole process was much less stressful than I expected but still something I do not want to do again.  I am really looking forward to getting to the microscope and starting to count the samples that I picked up from NOAA.  In addition to counting samples I will be working on designing some rapid deployment mooring for putting out thermistors and an ADCP this summer while I am sampling internal waves.  I look forward to sharing some of the interesting creatures I find in the samples with everyone.

 

under: Leif Rasmuson, Robert E. Malouf Marine Studies Scholar

Wow where did the term go!?!?

Posted by: | December 12, 2011 | 1 Comment |

Well the term ended and it was a whirl wind.  The majority of the term was spent either TAing biological oceanography or studying and preparing for my oral exam.  The TAing went extremely well and I really enjoyed getting to hear the lectures again.  Hearing the lectures really helped me to feel more prepared for my upcoming orals.  For part of my project I am using plankton samples collected by NOAA to better understand the vertical migratory patterns of Dungeness crabs.  This quarter I went to pick up the samples from Newport and they filled the back of a pickup truck!!!!  It definitely seems I have my work cut out for me.  However, the majority of the quarter has been spent preparing for orals which has entailed reading countless scientific articles.  Before the oral exam I have to turn in a written form of my proposal so the next few weeks will be spent finalizing this.  I am looking forward to getting past the orals portion of my PhD so I can get to work counting samples and building my computer model.  I will be posting sometime in February to update everyone on how the orals went.

under: Leif Rasmuson, Robert E. Malouf Marine Studies Scholar

Hi All,

IMG_1150

My name is Leif Rasmuson and I am graduate student at the University of Oregon’s Oregon Institute of Marine Biology, OIMB.  I am just beginning my second year as a PhD student in Dr. Alan Shanks lab.  I am interested in how hydrodynamics affect the larval stages of commercially important marine invertebrates. For my PhD I am focusing my research on the larvae of the Dungeness crab, Cancer magister.

Dr. Shanks has been using a light trap (see the image below) to capture Dungeness crab megalopae for 10 years.  In Dr. Shanks’ work he has found a strong correlation between the number of returning larvae and the commercial catch four years later.  The number of returning megalopae was found to be negatively correlated with the date of the spring transition, which is an atmospheric shift in winds causing the ocean currents to shift from southerlies to northerlies.  With this change in winds comes a seasonal switch from downwelling favorable conditions to upwelling favorable conditions.  Furthermore, in Dr. Shanks work a negative correlation was found between the number of megalopae recruiting and the Pacific Decadal Oscillation (PDO).  The PDO is decadal oscillation in water temperature from warm to cold temperatures off Alaska caused by variation in the flow of the West Wind Drift as it enters the California Current.  During cool, or negative phase years, more cool water is shifted into the California current causing the southward flowing California current to be stronger.  In negative phase years there were far more megalopae caught in the light trap (about 20 times more), larval return was very strong, suggesting that the increased southward flow may strongly influence the dispersal of the larvae.

 

IMGP0574

 

For my PhD I intend to use an individual-based couple biophysical computer model to mimic annual hydrographic conditions to examine the correlations talked about earlier. The output from the computer model will be tested against ongoing and historic light trap data to ensure that the model is accurate and provides a realistic description of the movement of larvae.  Since the light trap has been in operation for ten recruitment seasons, I will be able to model many years of ocean currents and PDO regimes allowing me to ground truth the models with historic larval return data ensuring that it is providing an accurate depiction of larval movements.

Since I am just entering into my second year of my PhD I am in the process of preparing for my oral defense that will be occurring this coming fall. Unfortunately this means the majority of my time I will be reading and writing which do not lend themselves to interesting blog posts or pictures. However, I will be sure to post any interesting happenings that occur and fun pictures.

 

under: Leif Rasmuson, Robert E. Malouf Marine Studies Scholar

Categories