Reasons for which species have been introduced
•Aquaculture
•Sport
•Ornamental
•Improvement of wild stocks
–New food fisheries
–Fill a vacant niche
–Forage for predators
–Restoration of fisheries
–Establish a wild stock
•
•Accidental
–Escape or release from aquaria and bait bucket
–Diffusion through waterway
–Introduced with other species
–Live transportation for consumption
–Ballast water of ships
–Shipping water (zooplankton)
–Packing material (seaweed)
•Control of unwanted species
–Aquatic vegetation
–Mosquitoes
–Snails
–Blooms of phytoplankton
–Other fish (usually exotic)
–
–
•
Source: Welcomme 1988
There are almost as many ways for introductions to occur as there are introductions. But here is a list of the most well-known and documented. I found one example of shipping water very interesting.  Several years ago 3000 Pacific oysters were shipped from California to Woods Hole.  The shipment had 2 certificates stating it was parasite and disease free.  That was all well and good but only 10% of the shipment was checked which I believe is standard. After arriving at Woods Hole the water was examined that the oysters were shipped in. The oysters were also washed and the water saved. There was a total of 29 species of algae, diatoms, protozoans, and invertebrates detected.