The journal contains different information about the world around you as well as a log over different activities completed or in progress such as higher or lower prices on fish due to demands, various missions in progress or completed from the local pubs and so forth.
The seasons tab will tell you which type of fish is currently in seasons, which means it is the best time of the year to catch that particular fish type. During the season there will be more hotspots on the map for that fish type which makes it easier to catch it.
The different seasons for fish can also be viewed in the Fish Season Table in this wiki.
Sometimes there will be an increase in price for a particular fish, or a drop in the price despite the fish being in season. These types of events are logged under the Events tab in the Journal and it is important to check this as it can greatly affect the prices you get for fish (both in the negative and positive) as well as special events such as a particular fish breeding more than usual thus making it easier to catch large volumes of that fish type.
Sometimes when docking at a port there will be jobs available from the local pub in that port. These jobs are typically to carry a certain volume of fish from one port to another or to go out and catch a specific volume of a fish type and then deliver back to the port again. These jobs typically pay a lot more per kg of fish and is usually worthwhile picking up for the extra profit.
Your currently active jobs as well as completed ones will be listed in this Jobs tab. You may also cancel any ongoing jobs by simply clicking on the "In Progress" status.
Note: Some jobs will disable your ability to Fast Travel so read the job descriptions closely if you rely a lot on Fast Travel while playing.
The sales tab keeps track of all sales you have completed. It shows each fish type, the different weights (if applicable), the amount of fish caught and the amount of kg's caught with the listed sales prices. It also keeps track of any additional bonuses you got from the sale (i.e. bonuses from the reputation with the current port) and how much pay each crew member takes.
Under the Gear tab you will find a detailed overview of all the nets and lines currently active (in the water) as well as a historic log of how much fish you caught when you pulled the net or line, the position it was set at, what type of bait used (applicable for lines only) and so forth. This is a great way to keep track of how much you catch using different size lines and nets so you can estimate how much fish you will bring onboard and thus make sure you don't overfill your boats cargo capacity and waste fish.
The Certs page gives you a quick overview of which boats you are currently allowed to use as well as a progress bar which lets you know how many kilometers you still need to travel in order to unlock the next boat.